Chemical & Process Technology: Process Critical Line

Tuesday, April 13, 2010

Process Critical Line

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.

Gravity flow

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.

Pump Suction

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 Suction

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

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.

Flashing / Two phase Gas-Liquid Flow

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.

Liquid-Liquid Coalescer

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 Line

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.

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. 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.

Wet Corrosive Service

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.

Critical Pressure drop line

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.

Pressure Relief Valve Inlet

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.

Pressure Relief Valve Outlet

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.

Control valve and Restriction Orifice

Fluid passing control valve and restriction orifice continuously will generate low frequency noise. This noise wave will be transmitted to downstream piping and result Flow Induced Vibration (FIV) which potentially
leads to pipe cracking in particular at small bore connection to large line

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.

2 Comments:

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