Chemical & Process Technology

A process plant is in debottlenecking phase and a pump is expected to be changed out. A datasheet submitted by pump vendor and stated the impeller size is 295mm with pump head of 120m as requested. The pump speed is expected to be high speed pump with 2950 rpm. Do you think this impeller can deliver the head ?

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We have been talking about the benefits of Variable Frequency Drive (VFD) for pump in Many Aspects. However, no single rule govern all scenario. Similarly, use of VFD for pump may not apply to all application. Thus, Affinity laws is applied to show the performance varies with speed in order to decide if application of pump with VFD for a particular scenario is feasible and variable.

Read details in "When Should Variable Speed Drives be used to Save Energy?"

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Discussion in "How to predict Check Valve Slam ?" has shown potential of surge pressure if a normal check valve is provided in the pump discharge. Nevertheless, in many design the surge pressure could be within the allowable limit of piping short term pressure spike. Thus, it is "normally" that providing a normal check valve on pump discharge does not poses any danger of pressure surge. Having said this, a proper checking should be carried out.

One of the scenario that has been warned for many times is providing double check valves in the pumping system. With single normal check valve in the pump discharge line, the discharge may experience a short term pressure spike as shown in image below.

In case of double check valves in the pumping system, there may be a very short time gap between the closure of both check valves. This would allow both check valves generate two pressure waves running forward and backward along the pumping system. Two waves would probably meet along the piping. Both waves' amplitude could be added or subtracted between each and other subject to wave's pattern. A severe pressure spike could be additional of two identical waves as shown in the following image.

Above discussed about the potential of severe pressure spike in the pumping system with double check valves, however, it very subject to :

thus, providing double check valves does not mean severe pressure spike will definite happen. However, the potential of severe pressure spike increases as well as it associate risk. Only proper pressure surge study and review on the wave and pressure spike pattern can advise the likelihood of occurrence.

Don't weak-up the Sleeping Dragon in the East ! This is one of the talk being heard for many years. What this Sleeping Dragon in the East ? Yes...China ! Recently heard development in China was tremendously fast especially in large city such as Beijing, Shanghai and GuangDong. Although this fast development is a reality, however, there are still many peoples including engineer are still having problem in communication and reading in English. They are still prefer to read Chinese.

Chemical Engineering has recently published the CHEMICAL ENGINEERING Supplementary issue which has been written in Chinese. If you are can read Chinese wording, probably you will have another way to read Chemical Engineering. Click here or following image to read.


In addition to capture readers from China and attracts more advertisers interest in advertising in this magazine, Chemical engineering supplementary (in Chinese) may also attract many professional engineer in China publish their article in Chemical Engineering. This is really a great move by Chemical Engineering.

India is another country in fast development competing with China. When will be the release of Chemical Engineering in India language ? From marketing perspective, guess this probably will realize soon !

Professional engineer from India, what do you think ?

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The discussion on pump cavitation have been circulated around what is pump cavitation ? how destructive a cavitation ? how cavitation sound & looks like ? what the relationship between NPSHa & NPSHr ? how to increases NPSHa to minimize / avoid cavitation ?...

One of the important topic is real life operation, detection of pump cavitation during operation. A pumping system may be designed and constructed for pump cavitation free as well as normal operation. Whenever some parts within pumping system is deteriorated, fluid degradation, suction condition change, pump suction and impeller wear-&-tear, would lead to short term intermittent or long term cyclic or continuous pump cavitation. This cavitation may not easily be detected.

There are several symptoms of pump cavitation. During pump cavitation :

• Severe noise
• Drops in flow
• Power consumption increases
• vibration
• instable

Noise could be similar to some solid piece (i.e. rock) passing the pump, popcorn popping and particle hitting metal sheet. Click here to hear pump cavitation. According to researches, high frequency resonances in pump occurred during pump cavitation. Pumping flow may suffer partial drop and increases in power demand. However, these parameter may not be detected easily. Similarly, vibration and instable operation may not be detected without a proper device in place.

There are three ways to detect pump cavitation.

i) Visual & Noise detection.
An experienced operator standing beside a cavitating pump can fairly easily distinguish noise caused cavitation than other machinery noise. However, there are large amount of pumps in a plant, it is almost impossible to locate operator near pumps in order to identify intermittent cavitation.



ii) Detection by Vibration level monitoring
Another method to detect occurrence of pump cavitation is continuous monitor vibration level of the pump. The is one the mature method nowadays. The vibration level signal is transmitted continuously to condition monitoring system. It can be programmed to provide signal and alarm to alert operator. Following is vibration trend line shows pump normal operation, partial cavitation, full cavitation before it is shutdown.


The vibration level trend line can be available for preventive maintenance analysis. This method can provide information on early detection of potential of pump cavitation so that maintenance group can take early actions.

iii) Detection by Acoustic Emission (AE) analysis
As high frequency noise is emitted during cavitation, Acoustic Emission (AE) technique in condition monitoring of rotating machinery can be used to detect pump cavitation. AE technique basically providing a AE detector several feet away from the pump to capture acoustic level.

Researcher reported that certain carbon nanotube structure (long, thin carbon nanotubes) might cause cancer for long exposure within sufficient quantities inhaled. Carbon nanotube (long & thin) has some properties close to asbestos which potentially cause mesothelioma, a cancer of the lung lining that take 30 years to manifest.

The good news is short carbon nanotube may not pose similar cancer risk than long, thin carbon nanotubes.

This is one the development where all new stuff will have to be gone through as we understood it more and more. Although it pose some level of risk, however sufficient precaution and procedure in place shall be able to minimize and avoid the consequence. General saying : Risk can not be avoided but can be managed. Those who are advancing their interests and works in nanotube study and production is encourage (shall) to take quick actions.

Google, well known giant internet search engine provider, is now planning into the energy business. Its main headquarters in Mountain View, California is being powered solar energy, as understood it is one of the largest solar electric installations in the United States.

As part of the continuous move into renewable energy, Google has planned to invest $20 million in the next year toward renewable energy research. The move include hiring between 20 and 30 new employees and renewable energy experts.

The main goal for present move is to find a great way to reduce the cost of renewable energy generation to bring it more on par with the cost of coal.

Varying opening of flowing path area in valve would be able to limit quantity of fluid passing the valve and serve the throttling and control purposes. Valve used for throttling and control purposes generally are globe, needle, special designed control valve, etc.

Valve also use for system overpressure & vacuum protection. For overpressure protection, fluid is contained in system during normal operation. Whenever the system pressure increase, the valve will open to relief fluid to ensure system pressure below maximum allowable working pressure of the system. Valve use for his purpose is generally spring loaded pressure relief valve and pilot-operated pressure relief valve. Similar principle apply to vacuum protection but the difference is only allowing fluid enter into system to avoid vacuum.

Valve like ARC valve is a self-contained valve which contain the check valve feature while allowing flow diversion during low flow. This is a good combination in pumping system where it demand back flow protection to avoid impeller damage and minimum flow protection during low flow.

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"Vacuum Hazard - Another Catastrophic Factor..." discuss about the damage of vacuum, main factors resulting Vacuum and several scenario may lead to Vacuum Hazard. "4-steps Approach to Combat Vacuum Hazard" discuss about approach may be taken to minimize and avoid Vacuum Hazard.


The following video clip will demonstrates how powerful is ATMOSPHERIC pressure and how quick a "vessel" collapse.


* Click here to view video clip via browser...

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A centrifugal pump feeding liquid to a heat exchanger located downstream. Check valve is provided between pump and heat exchanger to avoid back flow damaging the pump impeller. As the heat exchanger design pressure is lower than the pump shut-in pressure, A pressure relieve valve (PRV) is provided just upstream of the heat exchanger to avoid overpressure during pump shut-in condition. A shutdown valve (SDV) is located downstream of the heat exchanger for system isolation purpose. Both heat exchanger and SDV are located at one deck (~ 10m) higher than the pump. In the event of system shutdown, the Emergency Shutdown System (ESD) will initiate shut close of SDV and pump stop. Above arrangement is pretty standard in Chemical plant or Oil & gas plant.

When the system is commissioned and put under operation, everything run smooth. Nevertheless, the PRV open and slam close whenever the system shutdown. What was the problem ?


Why PRV passing during shutdown?
After some level of analysis, the following were postulated :

i) Whenever the system (pump & SDV) shutdown, the closure of SDV and lead to incompressible liquid back flow to check valve (in wave motion). The check valve slam (ordinary check valve) and lead to forword wave . The forward and backward wave would subsequently lead to pressure spike in the line and resulting opening of PRV. Read "How to predict Check Valve Slam ?" to get more insight into how ordinary check valve lead to pressure spike.

ii) As the heat exchanger and SDV is located ~ 10m above the pump & check valve, whenever the pump stop and check valve closure, liquid column would induce back flow and further increase the pressure spike.

iii) Pump rotate in forward direction would have substantial inertial and take longer time from full speed to zero speed. However, SDV would have much shorter closing time from full open to full close position. Apart from pressure spike caused by the ordinary check valve & SDV, the reducing forward flow would further increase the potential of pressure spike.

All aboves lead to pressure spike and potential lifting PRV.


How to avoid ?
Few approaches may be considered to minimize / avoid above situation :

a) Conduct surge analysis and check the potential of pressure spike

b) Use Non-Slam Check valve (if requried). Read "How to predict Check Valve Slam ?"

c) Increase SDV closing time. Read "Maximum Allowable Time from Shutdown Initiation to SDV Fully Closed" for maximum SDV closing time.

d) Use surge suppressor.

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Pressure Relief Device (PRV) inlet line lose shall be limited to 3% of set pressure while the outlet built-up back pressure shall be limited to 10% for a conventional PRV and 30 for balanced PSV (without affecting it flow).

Excessive PRV inlet loss can result rapid opening and closing of the valve which normally called chattering. Chattering will result in lowered capacity and damage to the seating surfaces. The inlet line loss here is referring to non-recoverable entrance losses (turbulent dissipation) and frictional line lose.

The impact of excessive built-up back pressure on discharge piping discussed in "Several Impact of Backpressure on Conventional PRV".

One of the question raised is that should the rated flow or required relieving flow to be considered for inlet line loss and discharge built-up backpressure estimation ? Should the flowing fluid phase affecting this requirement ?

Flowing phase & PRV type
The flowing fluid phase would affect the PRV type. Example, pop action spring loaded PRV is good for compressible vapor or gas service and modulating type PRV is good for incompressible liquid service.

Flowrate to be considered for Discharge line
As per API Std 521, section 7.2.1, table 12, the flowrate to be considered for tail pipe, lateral & main header subject to PSV type (refer table below).


Above is inline with API RP 520 PII - Ed 5 Aug 2003, section 5.3.

"...The rated capacity of a conventional spring loaded, balanced spring loaded or pop action pilot-operated pressure relief valve should typically be used to size the atmospheric vent piping or the discharge line from the pressure-relief valve to the relief header. Common relief header piping in closed discharge systems should be sized using the protected system's required relieving capacity.

For a modulating pilot-operated pressure-relief valve, the discharge piping can be sized using the required relieving capacity of the system that the valve is protecting..."

Flowrate to be considered for Inlet line
Following API Std 521, section 7.2.1, table 12, the inlet line loss for Modulating type PSV may consider required relieving flow. However, API RP 520 PII - Ed 5 Aug 2003, section 4.2.2. stated :

"When a pressure-relief valve is installed on a line directly connected to a vessel, the total non-recoverable pressure loss between the protected equipment and the pressure-relief valve should not exceed 3 percent of the set pressure of the valve except as permitted in 4.2.3 for pilot-operated pressure relief valves. When a pressure-relief valve is installed on a process line, the 3 percent limit should be applied to the sum of the loss in the normally non-fowing pressure-relief valve inlet pipe and the incremental pressure loss in the process line caused by the flow through the pressure-relief valve. The pressure loss should be calculated using the rated capacity of the pressure-relief valve."

API RP 520 PII - Ed 5 Aug 2003 has not clarified if required relieving flow can be used for modulating type PSV.

2-cents opinion
Modulating type PSV is intended to work to modulate the flow passing the PSV. Those theoretically, on the required relieving flow should be passing the PSV. In taking conservative approach and in compliance with API recommendation, rated flow is recommended for modulating type PSV inlet line loss estimation.

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16 organizations have formed European Observatory on Nanotechnologies with the objectives to provide reliable and complete scientific & economic information of nanotechnology to support european decision maker (e.g. government, industry, etc).

16 organizations are :

They are main from UK, Germany, France, Netherland, Finland, Spain, Italy, Switzerland, Denmark and Czech Republic.

IEM has released the IEM 49TH ANNUAL REPORT for 2008.

Vapor Liquid separation can be simulated in HYSYS using 2-phase separator. Following is an example with below feed gas at 32 barg @ -30 degC feeding a vapor liquid separator. Feed gas composition are N2 (0.38%), CO2 (0.68%), C1 (90.4%), C2 (4.1%), C3 (2.6%) and nC4 (1.84%), all in mole percent. Demister is provided in the vapor phase and maximum allowable pressure of 15 kPa is assumed. Those would like to understanding details of demister, read more in "Quick Understanding & Estimation of Mist Eliminator in Gas-Liquid Separator".

Why liquid in Vapor outlet ?
Above has been simulated HYSYS. It is pretty simple using 2-phase separator module in HYSYS. One of observation is the vapor outlet stream contains liquid. Vapor outlet from 2 phases separator should be in vapor phase. Why HYSYS predict present of liquid in vapor outlet ?


When pressure drop is allocated on the demister by entering pressure drop at "Vapour outlet", HYSYS view it as pressure drop on the vapor stream, outlet of 2-pases separator module. Vapor from separator is saturated vapor and pressure drop on the saturated vapor would result condensation due to Joule-Thompson (JT) effect. A liquid generated will be carried into the Vapor stream.

How to properly simulate a separator ?
In real separation, liquid generated in the demister (due to JT effect) will coalesce, formed larger droplet, trap by demister, droplet is separated from vapor leaving the separator and drop into liquid phase by gravity force. Thus, above straight forward simulation does not really represent a true separation. A proper simulation may include a "dummy separator" with zero pressure drop, downstream of the vapor outlet (2 phases). Separated liquid is mixed with feed stream using a "dummy pump". With this method, the vapor outlet from "dummy separator" will represent the true vapor from the separator. In this method, the "dummy separator" represent the demister and "dummy pump" represent the gravity form.


Above is just a simple trick for simulation and many may ignore it or do not even aware of this phenomenon. However, it is pretty important in presenting a true & correct Heat & Material Balance.

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