Chemical & Process Technology

Earlier post “Process Critical Line” has presented a checklist of process critical line. During design phase, these process shall be checked in detail to minimize or avoid problem such as vibration, hammering, capacity reduction, cavitation, etc to occur. This post will further present good engineering practice for process critical line.

Gravity Flow
Any line subject to gravity flow e.g. drain, flare, vent, etc, low pocket shall be avoided. Liquid or solid accumulate in low pocket potentially result corrosion and blockage. Line should be sloped (and/or free draining) from sources to receiver.

Pump Suction
Line to pump suction should be designed to allow self floating as far as possible where lowest liquid level is above the pump highest point.

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Ensure minimum submergence of tank to avoid potential vapor being sucked in to pump suction line due to vortex. If positive submergence is not achievable, installation of vortex breaker is another option. Read more in “Vortex Breaker to Avoid Vapor Entrainment”.

Any high pocket shall be avoided and provision of eccentric reducer at the pump suction to avoid potentially vapor lock prior to pump start-up.

Ensure NPSHa is always higher than NPSHr with a positive margin e.g. 1m for entire operation range (turndown to design capacity) and operation conditions (highest operating temperature). There are 17 Ways to Reduce Likelihood of Pump Cavitation.

Minimize suction length and fitting as much as possible to minimize potential of pump cavitation.

Compressor Suction
Compressor suction knock out drum (KOD) may be equipped with mist eliminator e.g. wiremesh to promote droplet coalescing and separation.

KOD vapor exit nozzle should be designed large enough to minimize exit momentum (rho V2 less than 6000 Pa) in order to minimize reentrainment of coalesced liquid droplet into vapor.

Compressor in general can tolerate small amount of liquid. If absolute no liquid is allowed enter compressor as imposed by compressor manufacturer, one may consider provision of insulation to minimize ambient and JT cooling and heat tracing to compensate heat loss due to above mentioned cooling.

Absolute no low pocket shall present in the compressor suction line as low pocket can accumulate liquid and slug of liquid can cause severe damage to compressor.

May consider a compressor suction strainer for start-up and commissioning. As compressor is sensitive to suction line pressure drop, any additional fitting and device at compressor suction can lead to capacity reduction, installation of suction strainer shall be analyzed in detail during design phase.




Flashing / Two phase Gas-Liquid Flow
Slugging and plugging flow in vertical and horizontal potentially results significant vibration to piping. During process design phase, slugging and plugging flow shall be avoided for entire operating range (turndown to design capacity) and operating conditions.

May consider provision of vapor liquid separation and run separator separate header for vapor and liquid line if slugging / plugging flow is unavoidable. For steam header, provide sufficient steam traps to drain-off condensate and minimize potential of slugging flow.

Extra and strengthen support may be provided to avoid severe vibration and failure on pipe crack.

Liquid-Liquid Coalescer
Vapor generation in liquid-liquid coalescer may accumulate and result under-performed liquid-liquid separation. May consider to provide sufficient static head to suppress vapor generation in liquid-liquid coalescer. It is always recommended to provide a vapor equalization line back to separator to release any vapor form in liquid-liquid coalescer.

Low Pressure Line
Minimizing pressure drop in low pressure line is the key factor to ensure proper performance of system. Minimize line length, fittings, elbow, etc and use of smooth surface pipe e.g. stainless steel may be considered.

Potential Surge Line
Steam supply line experience heat loss and condensation due to partially damaged insulation and extreme low ambient temperature. Flashing condensate with steam return to collection header mix with cold condensate. Both condition would results sudden steam collapse and lead to implosion. Steam implosion would generate severe movement of condensate in the collection header and severe vibration of header. Therefore proper maintenance of insulation is extremely important in keep steam line from transient surge. Besides, provide sufficient steam trap to eliminate condensate from steam line.

Long pipeline transferring incompressible fluid e.g. LNG rundown line, produce water injection line, etc potentially experience transient surge (water hammer) in the event of closure of shutdown valve. Transient surge analysis shall be conducted during design phase to ensure surge is avoided. Slower closure of shutdown valve is one of the key component in minimizing surge in long pipe line. Non-slam check valve on the pump discharge may also assist in minimizing surge in long pipeline with pump. Surge suppression system may be considered in the event surge is unavoidable. One shall take note that provision of pressure relief valve may not help to eliminating surge due to slow response time of PRV.



Pressure Relief Valve Inlet & Outlet
May consider discussion and recommendation in :

• Concerns & Recommendations on PSV INLET line 
• Few Concerns & Recommendations of PSV Discharge Tail pipe

Control Valve & Restriction Orifice
Flow Induced Vibration (FIV) and Acoustic Induced Vibration (AIV) may be studied to identify location of piping which potentially experience high risk of low frequency and high frequency vibration. Minimizing small bore connection may be considered e.g. provision connection with more than 2 inches, avoid using connection smaller than 2 inches. For small bore connection, may consider brazing and extra support to strengthen the connection and avoid pipe cracking.

Anti-cavitation trim could be considered for control valve potentially experience cavitation. Similarly provision of multiple restriction orifice (RO) in series or multi-ported RO may be considered if cavitation occurs in RO.

Process design involve line sizing and pressure profile definition. All line size will be presented in Piping & Instrumentation Diagram (P&ID). Nevertheless, there is no line length, elbow  and elevation define in P&ID. Upon receipt of P&ID, Piping engineer will begin the piping routing activities and assign necessary length, elbow and elevation to the line. This piping routing may not consistent with assumption taken by process engineer during earlier process design. Significant increase in pressure drop, wrong routing of pipe , incorrect sloping, etc could lead to severe vibration, valve chartering, reduced capacity, under-perform equipment, etc. Therefore it is important for a process engineer to identify Process Critical Line for detail isometric checking. Following will tabulate typical line may experience potential problem and required  detail process checking.

In general most fluid is transferred by pressure from source to destination during normal operation. Pressure head available at source will overcome frictional loss, velocity head and static head. This allow fluid transfer from low point to elevated point. Typical example is transfer liquid from closed drain drum to production separator with the pressure head develop by a reciprocating pump. This kind of pipe is typically know as pressurized pipe. Nevertheless, there are some fluid is transferred by gravity force (or static head). Typical system is closed drain network, process line designed for gravity transfer, etc. Improper design of gravity flow would lead to reduce or no flow.

Cavitation is phenomenon cause by bubble generation follow by bubble collapsed. More thorough discussion on cavitation phenomenon, cavitation damages and the way to minimize / avoid cavitation can be found in following post :

Typically to minimize / avoid cavitation damage is to ensure Net Positive Suction Head required (NPSHr) by the pump is lower than the NPSH available (NPSHa) by the system itself. Pump suction line size and routing is a dominant factor affects NPSHa. Improper design of pump suction line would lead to severe cavitation, vibration and pump damage.

Centrifugal Compressor capacity is subject to designated flow and compressor inlet condition. Any changes in suction condition (e.g. decrease in density) would seriously affect compressor capacity (e.g. decrease in capacity). Improper design of line between Compressor suction Knock-out drum (KOD) and compressor inlet nozzle would lead to high pressure drop, subsequently lower density and capacity decrease. 

Long compressor line (from KOD to compressor) would increase potential of heat loss to ambient (severe during winter time) and results condensation. Present of condensate in vapor to compressor and impinge on compressor impeller when vapor is accelerated potentially damage compressor impeller and severe vibration in compressor.

Flare / vent collection header has significant impact on built-up backpressure to pressure relief valve. (PRV) Severe pressure drop can lead built-up back pressure exceed it allowable limit e.g. 10% for conventional type PRV. Warm fluid mix with cold fluid in flare header may results two phase gas liquid flow in flare header. Similarly, severe flare header vibration can occur with the present of slugging / plugging flow.  Low point in flare line potentially results liquid accumulation in flare line and corrosion may occur. In the major relief event, high velocity vapor pushing accumulated liquid would results slugging flow in the flare line. Liquid column flowing at vapor velocity knocking of elbow/bend may generate severe vibration.

Liquid at saturation point coming from separator potentially flash and two phase gas liquid flow. Typical flow regime is Bubbly flow. Similarly saturated vapor experience ambient cooling and line frictional loss results condensation and two phase gas liquid flow. Typical flow regime is Mist flow. Both Bubbly and Mist flow are not destructive in nature and properly a normal support would be sufficient. Nevertheless, slugging and plugging flow in vertical and horizontal potential results significant vibration to piping. Extra and strengthen support is required to avoid severe vibration and failure on pipe crack. More discussion on Problems Caused by Two Phase Gas-Liquid Flow.

Saturated liquid from separator feeding liquid-liquid separator, any pressure drop increase potentially lead to vapor accumulation and  under-performed liquid-liquid separation.

Low pressure stream e.g. overhead from amine regeneration column, end flash gas from end flash column, etc is very sensitive to frictional loss.Low pressure here is pressure very close to atmospheric pressure. Any increase in frictional loss will seriously reduce flow through the pipe.

Steam supply line experience heat loss and condensation due to partially damaged insulation and extreme low ambient temperature. Flashing condensate with steam return to collection header mix with cold condensate. Both condition would results sudden steam collapse and lead to implosion. Steam implosion would generate severe movement of condensate in the collection header and severe vibration of header. Long pipeline transferring incompressible fluid e.g. LNG rundown line, produce water injection line, etc potentially experience transient surge (water hammer) in the event of closure of shutdown valve. Piping surge is severe in nature and potentially lead to pipe crack and support failure.

Some line is normally flow with vapor contains CO2 & H2S and sulfide stress corrosion cracking (SSCC) and general CO2 corrosion is not expected as only vapor flow. During winter low ambient temperature and under turndown operation, ambient cooling potentially lead to vapor condensation and induced SSCC and general corrosion on under-designed piping. Typical example is Condensate stabilizer overhead. Similarly warm wet flare header is normally dry due to continuous dry gas purging. In the event, PRV passing leaks wet vapor into warm wet header or any PRV open follow by closure of PRVs, wet vapor potentially condensed and accumulate in low point and results general corrosion.

Line normally design for low pressure drop, any increase in pressure drop could to capacity reduction and/or under-perform downstream unit. Typical example is high pressure gas feeding liquefaction Main Cryogenic Heat Exchanger. Any reduction in Feed pressure to MCHE would lead to higher heat of vaporization and reduce LNG production.

Under normal design condition, PRV inlet line non-recoverable pressure loss shall be limited to 3% of PRV set pressure  Any significant increase in line length and elbow (due to piping routing) will results non-recoverable pressure loss increase and lead to PRV chattering. 

Upon opening of PRV, instantaneous large gas or vapor passing PRV. High frequency noise is generated results acoustic induced vibration (AIV)  which potentially cause discharge pipe cracking. Instantaneous large gas/vapor flow accelerated from zero velocity to maximum velocity will induced high reaction force to downstream piping. Under-designed pipe may crack on high reaction force.

Saturated liquid passing a control valve or restriction orifice, pressure will began to decrease and lowest pressure closed to vena contracta, follow by pressure recovery once is passed the vena contracta. Lowest pressure point could be lower than vapor pressure of fluid. Vapor bubble will begin to form and once fluid passed through the vena contracta, vapor will start to collapse and results jet wave impacting control valve or restriction or piping. Above phenomenon generally known as cavitation which generate severe vibration to the piping.

Any scenario is normally ignore or miss by engineer where control valve downstream piping may not design for occurrence sonic flow downstream piping. This typically occur in line with control valve discharge to flare/vent header. Sonic flow potentially reduce flowing capacity and how reaction force to piping.

The performance of a control valve is defined by its inherent and installed characteristic curves. The inherent characteristic curve is a plot of the percent of valve opening vs. the percent of maximum flow coefficient (CV). The inherent characteristic curve is determined by measuring the flow rate at various positions of valve travel with a fixed differential pressure across the valve (typically 1 psid) and calculating the CV at each position using a form of the generalized Control Valve CV equation

Thanks to Jeff Sines

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Control valve has been widely used in Chemical & Process Plant for material feeding control and operating condition control, material feeding control. Today control valve technology is highly reliability and availability and long life span. It is a very mature technology. There are many handbooks and articles available online and FREE for read and download. In the past a control valve related documents listed in "Useful Documents Related to Control Valve". This time i would like to highlight a few that i think they are rather complete and you shall not miss them.

Handbook For Control Valve Sizing
Besides two most important handbooks for control valve available free in the net, Parcol is also offering Handbook for Control Valve Sizing which is brief but informative in nature.

Valve Selection Essentials

Valve failures, replacements, repairs, downtime and lost product can be greatly minimized by selecting the right valve, the first time. Great. So how do you assure you are selecting the right valve? There are basically 3 avenues:

1. Evaluate your system criteria relative to each valve type.
2. Utilize the expertise of a consultant or the factory Technical Sales Representative.
3. Utilize one of the new Valve Selection Software Programs provided by various valve manufacturers.

Read more in Valve Selection Essentials

Almost any type of valve can be used for control by fitting an actuator and positioner, though care must be taken to ensure that there is no excessive backlash present and it will be recognised many will not exhibit a good characteristic for precise control. A simple comparison table is presented in this article for easy selection.

Control Valves are the most important element of a fluid handling system and proper selection of these valves is crucial for efficient operation of the process. When sizing butterfly valves for control, it is imperative to have certain requirements of the system. Maximum flow requirement would be equivalent to the design flow and provided or converted to gallons per minute and Maximum pressure drop allowed where typically 3 to 5 pounds max. However, the pressure drop should never exceed one half (1/2) of the inlet pressure. Without these two factors, selection of a control valve would be simply a guess...

Selecting a Control Valve
Fluid velocity in a control valve is a key parameter that must be considered when sizing and selecting a control valve. High velocity can lead to erosion damage, trim wear, trim component failure, vibration and high noise levels. Therefore, it is vital to design for valve velocities within acceptable limits so that these problems are avoided. A maximum body inlet velocity of...

Compressor Anti-Surge Control Valves
Surge Control Valves must be capable to operate the compressor below the surge control line. The surge control line is always depicted to the right of the surge limit line in the compressor curves (maps). Most information required for the sizing of the surge control valves is available on the compressor map. As the compressor suction pressure may vary, various calculations need to be made...More recommendations found in this article.



Butterfly Control Valves
Butterfly valve with large Cv compare to globe type control valve is commonly used for low pressure drop and low pressure rating system. It allow large flow passing through and induce low pressure drop. This article will present some facts and characteristic about Butterfly control valve...

Valve Sizing Info
Valve size often is described by the nominal size of the end connections, but a more important measure is the flow that the valve can provide. And determining flow through a valve can be simple. This technical bulletin shows how flow can be estimated well enough to select a valve size—easily, and without complicated calculations. Included are the principles of flow calculations, some basic formulas, and the effects of specific gravity and temperature. Also given are six simple graphs for estimating the flow of water or air through valves and other components and examples of how to use them.

Control valve has been widely used in Chemical & Process Plant for material feeding control and operating condition control, material feeding control. Today control valve technology is highly reliability and availability and long life span. It is a very mature technology. There are many handbooks and articles available online and FREE for read and download. Besides two most important handbooks for control valve available free in the net, Parcol is also offering Handbook for Control Valve Sizing which is brief but informative in nature.

This handbook covers :

Download Handbook for Control Valve Sizing

Cavitation damage and fatigue due to acoustically induced vibration have discussed several times in previous posts such "cavitation" and "AIV". Control valve is known as one the common element / component in a plant potential source of cavitation and AIV related problems. Many efforts in combating both issues were proposed.

Typical solution is anti-cavitation trim e.g. staged trim, multi-flow path trim, labyrinth-disk type trim, etc.

i) Staged trim low noise
Staged trim low noise trim is adopted multi-stage pressure letdown similar to multi-stage RO.




ii) Multiple flow path type low noise trim

Detail construction of multiple path low noise trim (red circle) shown in below image. Typically it adopt the simple principle as use in RO. Multiple flow path follow by multiple expansion stage.



Read more in "Fisher® WhisperFlo® Aerodynamic Noise Attenuation Trim".

iii) Labyrinth-disk type low noise trim
Another type of low noise trim is the Labyrinth-disk type low noise trim. It works approximate the same way as Multiple flow path type low noise trim.




Above trims design is commonly based on general principle in cavitation prevention which is ensure the operating pressure along the flow path in valve trim above fluid vapor pressure. See below image.



Below are some old useful articles related cavitation and multi-stage disc trim available for download. The post part of the continuation post from "Useful Documents Related to Control Valve"


Fluid kinetic energy as a selection criteria for control valve
A selection criteria is provided that assures a control valve will perform its control function without the attendant problems of erosion, vibration, noise and short life. The criteria involves limits on the fluid kinetic energy exiting through the valve throttling area. Use of this criteria has resolved existing valve problems as demonstrated by retrofitting of the internals of many valves and vibration measurements before and after the retrofit. The selection criteria is to limit the valve throttling exit fluid kinetic energy to 70 psi (480 KPa) or less.

Multi-stage valve trim retrofits vibration eliminate damaging
Through the RHR valve trim retrofit at Quad Cities with multi-stage, tortuous-path, pressure reducing disks and an emergency capacity cage, the damaging vibration previously experience during system test operation has been eliminated. Further, an unlikely repetition of the previously experienced valve blockage by a Rad bag or any other medium has been precluded by the 50% over-capacity cage in the last 20% of valve stroke. Also, previous concerns regarding possible piping fatigue failures within the RHR system as a result of past severe vibration problems have been eliminated.

Specifying control valves for severe-service applications
Large number of the process control valves used in fossil-fired power plants must operate under severe-services conditions—that is, in high-pressure and / or high-temperature applications. When specifying valves for such applications, extreme care must be taken to avoid costly premature failures. This article discusses the stringent requirements that valves must meet to safety operate and deliver long-term performance under severe service conditions. Requirements are examined for both generic and specific applications.

Impact of control valve design piping vibration
Vibration of the recycle piping system on the main oil export pumps from a platform in the North Sea raised concern about pipe breakage due to fatigue. Failures had already occurred in associated small bore piping and the instrument air supply lines. and control accessories on the recycle flow control valves. Concern also existed due to the vibration of non-flowing pipe work and systems such as the deck structure, cable trays and other instrumentation, which included fire and gas detection systems. The vibration was finally solved by changing the control valve to a trim that added enough pressure stages to assure the trim exit velocities and energy levels were reduced to levels demonstrated historically as needed in severe service applications. This vibration energy reduction was more than 16 times. This was achieved by reducing the trim exit velocity from peaks of 74 m/s to 12 m/s.

Special thanks to Control Component Inc.

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Condensate Pump Recirculation Valve Selection
Power plant condenser receives exhaust steam from the low pressure turbine and condenses it to liquid for reuse. Condenser back pressure range from 1.0 to 4.5 Hg absolute (3.4 to 15.2 kPa) with higher pressures possible when the cooling water temperature is elevated by using an air-cooled steam condenser. Condensate is collected in the bottom of the condenser in the hot well. The condensate feed pump supplies the subcooled water to the feedwater heaters.

As with most centrifugal pumps, the condensate pump is subject to overheating and cavitation if used at a flow under a minimum value recommendation by the pump manufacturer. When the flow required by the deaerator level control loop falls below this minimum recommended value, additional flow is recirculated back to the condenser by opening a valve installed in a bypass line thus maintaining the minimum flow through the pump at all time. There are problems associate with this valve and its selection. Read more...

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High pressure feedwater pumps are subject to overheating and subsequently very rapid damage if used at low flow as compared to the rated capacity. The minimum flow required for pump protection is specified by the pump manufacturer. It is never less than 15% and can sometimes be 40% or more. When the flow required by the boiler is below this limit, the feedwater pump flow demand is artificially increased by discharging to the deaerator or sometimes to the condenser through a recirculation valve. The recirculation valve is required to operate either on-off within a selected range of values of flow to the boiler, or in modulating service. In this case, the flow through the control valve is equal to the difference between the pump minimum flow and the actual flow to the boiler. Modulating service avoids the waste of energy since the recirculated flow is kept at the minimum acceptable value, but it is more severe in terms of valve service.

Several precautions may have to be taken while selecting this type of recirculation valve :

1. Cavitation
2. Pressure drop distribution
3. Axial / Radial design
4. Shut-off/ Wire drawing
5. Clogging
6. Vibration
7. Failure position

Read more...

Control valve capacity (Cv) for particular application is determined by the use of recognized valve sizing equations. This valve equation can be found in several handbooks i.e. Fisher, Masoneilan, etc as discussed in "Useful Documents Related to Control Valve".This article presented a simple idea why a control valve is normally operate at around 60-70% valve opening and it associated impact such as increase signal dead band effect and affect optimum controller settings-wider proportional band and faster reset. It recommended a new way to overcome the impacts by introducing VARIMAX. Read more...

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Product fluid in reactor involve exothermic process is commonly hot. It is then sent to distillation and separation system for catalyst and raw material recovery. Separated product is then cooled by plant wide Cooling Water (CW) before it is sent to storage tank. The product temperature will have to be maintained.

How this temperature is controlled ?


Temperature Control Methods
There are several ways to maintain the product temperature :

(i) Provide a product bypass across the Cooler, control valves on Product bypass line and Outlet line (Split range control) with fixed CW flowrate
(ii) Provide a CW bypass across the Cooler, control valves on CW bypass line and CW inlet to Cooler with full product flow across cooler
(iii) Provide a CW bypass across the Cooler, control valves on CW bypass line and CW outlet to Cooler with full product flow across cooler
(iv) Provide a Control valve at the Cooler inlet with full product flow across cooler
(v) Provide a Control valve at the Cooler outlet with full product flow across cooler

Generally the product flow is fixed by operator based on production plan and the product flow shall not be controlled. Thus, it is always not recommended to provide a control valve at the inlet and outlet of product line for product temperature control.

Disturbance of CW Network Balance
Cooling water is in a network supplying to many heat exchanger through out the plant for cooling purpose. It is normally supplied by a set of centrifugal pump. As centrifugal pump head will be affected flow across, any changes in the CW demand will affect the CW balance in network. This will further affect the pressure in the network and hence the CW flow into other heat exchangers. Thus, it is always recommended not to throttle the CW flow as much as possible to avoid CW balance.

Scaling
Throttling CW flow into heat exchanger would potential lead to low CW flow into heat exchanger, high film temperature at on CW side and promote scaling. The option (ii) and (iii) are always recommended IF throttling on CW side is chosen.

Potential affecting Production
Controlling product fluid temperature with product bypass across the Cooler, control valves on Product bypass line and Outlet line (Split range control) and fixed CW flowrate (option i) is one of the common way in temperature control for product cooling. As it minimize the impact to CW network. Nevertheless, there is still concern about manipulating product fluid or mal-operation (controller failure) of control valves would potentially lead to production lost, the option (ii) and (iii) are always the recommended option.

CW Pressurise or Non-Pressurise
Option (ii) and (iv) compare to option (iii) and (v), the difference is the location of main CW line control valve (either at the inlet or the outlet). Providing a control valve at the outlet will have the following advantages :

a) Maintain high pressure in the heat exchanger and higher pressure will results higher heat transfer

b) CW at high pressure will minimise potential of boiling

c) CW at high pressure will minimise potential release of dissolved gases in CW , trap in heat exchanger and reduce heat transfer

d) In event of Control valve failure (failed to full close position), not further Cooling. CW in the heat exchanger will be heated and potentially lead to heat exchanger overpresure due to thermal expansion and/or boiling. The CW will be relieved via Pressure Relief Device provided on the heat exchanger. Providing control valve at the outlet would allow continue CW feeding into the heat exchanger, this minimise the potential of sudden temperature increase and cause heat exchanger due to thermal shock. The downside is release CW into disposal network.

Considering above advantages, it is always recommended to provide control valve on CW line at the outlet IF throttling CW side is chooses.

CONTROLLING SHELL AND TUBE EXCHANGERS
"Controlling Shell & Tube Heat Exchanger", an excellent article by Walter Driedger discussed about all type of control schemes around heat exchanger. Check out.

Related Topic

• Problems and Measures for Condensate Recycle Control Valve
• FAQ Related to Control Valves
• Useful Documents Related to Control Valve
• FREE & Reliable Control Valve Sizing Software
• Anti-surge Control (ASC) or Capacity Control (CC) Valve in Vertical Upward Run ?
• Combine Anti-surge control (ASC) & Capacity Control (CC) Functions ?

CONTROLLING SHELL AND TUBE EXCHANGERS
Shell and tube heat exchangers are among the more confusing pieces of equipment for the process control engineer. The principle of operation is simple enough: Two fluids of different temperatures are brought into close contact but are prevented from mixing by a physical barrier. The temperature of the two fluids will tend to equalize. By arranging counter-current flow it is possible for the temperature at the outlet of each fluid to approach the temperature at the inlet of the other. The heat contents are simply exchanged from one fluid to the other and vice versa. No energy is added or removed.

Since the heat demands of the process are not constant, and the heat content of the two fluids is not constant either, the heat exchanger must be designed for the worst case and must be controlled to make it operate at the particular rate required by the process at every moment in time. The heat exchanger itself is not constant. Its characteristic changes with time. The most common change is a reduction in the heat transfer rate due to fouling of the surfaces. Exchangers are initially oversized to allow for the fouling which gradually builds up during use until the exchanger is no longer capable of performing its duty. Once it has been cleaned it is again oversized...

"Controlling Shell & Tube Heat Exchanger", an excellent article by Walter Driedger discussed about all type of control schemes around heat exchanger. You may download here.

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Steam is commonly used in oil & gas, refinery, petrochemical and power plant for heating and power generation. Steam is condensed in equipment for heating and in turbine for power generation. Condensate is then collected in common collector before it is sent to condensate drum. Condensate from drum is then pumped to Boiler for steam generation via a Boiler Feed Water (BFW) pump. BFW is normally a centrifugal type and a minimum flow recirculation line is provided on BFW discharge for pump protection.


Minimum flow control can be

• a flow meter on pump discharge with control valve on recycle line
• a flow-Delta P and flow meter on pump discharge with control valve on recycle
• an automatic Recirculation Valves (ARC) valve

as discussed in "Centrifugal Pump Minimum Flow Control Strategies". Restriction orifice option is not normally used due to energy saving, avoidance of continuou noise and vibration.

Problems
There are several problems assocaited with these valves in condensate recycle line :

i) Erosion - flashing and cavitation results trim and body erosion
ii) Severe noise and vibration - flashing and cavitation
iii) Leakage - energy loss

Recommendation
Several recommendations to miniminse above mentioned problems :
i) Harden trim to resist erosion cause by flashing and cavitation

ii) Correct material i.e. alloy selection to avoid erosion-corrosion

iii) Anti-cavitation trim to minimise / avoidance of cavitation.

iv) Multi-stage anti-cavitation trim for small valve

v) Multi-hole anti-cavitation trim for large valve

vi) Multi cage anti-cavitation trim for high pressure recovery (FL) valve

vii) High lift (more than 20% lift) valve to increase trim life

viii) Large body valve to minimise velocity (high velocity lead to high erosion) in the valve inlet and outlet chambers. [Tips : Body erosion proportional to 3-5 power of velocity]

ix) Elevate condensate drum to increase back pressure to the valve (if possible)

x) Provide restriction orifice downstream of control valve to increase back pressure. One shall take note at low flow, the pressure drop acrosss RO is negligible. Majority pressure drop (energy being "killed") still occurred at valve

xi) Tight shut off (class V) valve to avoid leakage and hence energy loss.


Related Topic

• Understand Boiler Efficiency
• Simple Formula To Estimate Water Viscosity
• Steam - Condensate Useful Links...
• Square-root-Square-root Formula Ease Saturated Steam-Codensate Temperature Prediction
• Useful Steam - Condensate Calculator
• Steam in FIRE...

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Centrifugal compressor is widely used to increase process fluid pressure head to meet process requirement. All centrifugal compressors are equipped with Anti-surge control valve (ASCV) for equipment protective purpose. In many events, This valve may also serve as capacity control valve in order to maintain a specific process parameter.

One of the question being discussed :
Should we locate compressor Anti-surge control (ASC) or Capacity Control (CC) Valve in vertical upward run ?

Good engineering practice is to install anti-surge control valve in horizontal run and no low pocket along the inlet and outlet of the control valve to avoid any possible liquid (or solid) accumulation which possibly lead to issues like corrosion, liquid slug, liquid freezing, solid plugging, etc.

When ASCV / CCV in close position, vapor with mist liquid may diffuse along the inlet and outlet of ASCV/CCV. Due to heat loss to ambient and/or mist coalescence, mist is possible condensed and accumulated in low pocket and/or downstream of ASCV/CCV downstream on vertical upward run. If the fluid is wet and contains corrosive compounds i.e. Hydrogen Sulfide (H2S), Carbon Dioxide (CO2), low pocket and/or upward run where liquid is accumulated will experience acid corrosion. Pitting and crevice corrosion may be experienced.

A fluid with hydrate former and wet, when there is liquid accumulated due to external ambient cooling, there is potential risk of hydrate formation and water freezing. This potential partially or totally clog the recycle line.

Fluid Possibly Crystallization and Solidify
A fluid contain compound possible crystallize and solidify when it is cooled by ambient, when the fluid is cooled by ambient in the stagnant section in recycle line, the fluid is possibly accumulated at low pocket and/or vertical upward run, downstream of ASCV/CCV and crystallized or solidified and plugged the recycle line

ASCV/CCV is normally closed and liquid is accumulated along the recycle line due to external cooling. In the event of ASCV / CCV open to recycle vapor at compressor discharge back to suction, vapor will push the liquid column flow along the recycle line. This liquid column will be knocking at the ASCV/CCV, bend and tee. Severe slugging and piping vibration may occur and potential damage ASCV/CCV and line.

Thus, it is always recommended to install ASCV / CCV in horizontal run and no low pocket along the inlet and outlet of the control valve that potentially promote corrosion, slugging flow, liquid freezing and solid plugging.

Related Topic

• Combine Anti-surge control (ASC) & Capacity Control (CC) Functions ?
• FAQ Related to Control Valves
• Useful Documents Related to Control Valve
• FREE & Reliable Control Valve Sizing Software
• Understand Droop Effect in Self-regulated Control Valve
  
• WELKER Jet Contorl Valve Handbook
• How to apply valve equation in HYSYS Depressuring ?

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Control valve is widely used in Oil & gas, refinery, Petrochemical and chemical plant for control purposes of operating parameters i.e temperature, pressure, level, etc. Several articles as compiled in "Useful Documents Related to Control Valve" are pretty useful to many process and chemical engineers.

Centrifugal compressor is widely used to increase process fluid pressure head to meet process requirement. All centrifugal compressors are equipped with Anti-surge control valve (ASCV) for equipment protective purpose. In many events, This valve may also serve as capacity control valve in order to maintain a specific process parameter.

Anti-surge control valve (ASCV) is one of required equipment protective function for centrifugal compressor. It is used to protect centrifugal compressor from running in surge region, a phenomenon where centrifugal compressor discharge pressure high enough to results reverse flow follow by severe vibration in the compressor chamber. The Anti-surge control valve is recycling discharge gas back to suction to minimize differential pressure across the centrifugal compressor.

Capacity Control valve (CCV) is normally used to maintain a process parameter of a system by recycling excess gas back to compressor suction. Typical process parameters of a system are fix suction pressure, fix discharge pressure and fix differential pressure.

This is one of the most important factor to determine if separate control valve is required. For an anti-surge control purpose, the best control valve characteristic is quick opening. However, for capacity control purpose, an equal percentage is mostly used. Thus, this could easy lead to separate control valve with different characteristics used for dedicated function. However, in many occasion, the valve with equal percentage may also serve the purpose of anti-surge control. This is very much subject to compressor characteristic, system operating pressure, system volume, etc. Only way to prove if a single control serve two purpose is Dynamic simulation.

Safety
In many event, the Anti-surge control system together with anti-surge control valve are within a commercial package of compressor. The purpose is to have single point coordination and responsibility and to avoid unnecessary interference. However, using control valve dedicated to Anti-surge control purpose for capacity control purpose would lead to interference and additional signal managing the control valve. Some company safety principle do not allow an protection function used as control function as well. Thus, a separate / dedicate control valve is used. In many recent serious improvement in technology and working method, one control valve serving dual purposes are widely implemented recently.

Wear & Tear
For a control valve serving dual purposes, the control valve may continues in service and this promote wear and tear of the control valve. This would increase the downtime and availability of this control valve. Doubling the control valves will recover the availability again.

Cost
Dedicated control valve for ASCV and CCV is obviously require higher capital investment.

Used of dedicated control valve for anti-surge and capacity control purpose or combine function control valve is subject to case-by-case basis. No one key fit all locks...

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