8 replies to this topic

[QuantumEng]

Hello all,

 

I have seen various posts regarding the location of discharge for PSVs protecting piping/systems downstream of a PD pump for a blocked outlet scenario. Notable contributors such as Art Montemayor and fallah have stated that the discharge piping of the PSV should NOT be routed to the suction side of the PD pump.

Is there an explanation you guys can provide as to why this is not acceptable? Perhaps any documentation/source that supports this. 

I am in agreement with not routing the discharge to the suction side from a logical standpoint, but is there any further engineering evaluation behind it?

Many thanks in advance.

[latexman]

If depends. What’s the fluid? And, how stable is it, if heated via recirculating? And, how long will it take the pump recirculating to raise the fluid temperature to a unsafe temperature?

[fallah]

 

I have seen various posts regarding the location of discharge for PSVs protecting piping/systems downstream of a PD pump for a blocked outlet scenario. Notable contributors such as Art Montemayor and fallah have stated that the discharge piping of the PSV should NOT be routed to the suction side of the PD pump.

Is there an explanation you guys can provide as to why this is not acceptable? Perhaps any documentation/source that supports this. 

I am in agreement with not routing the discharge to the suction side from a logical standpoint, but is there any further engineering evaluation behind it?
 

 

Hi,

 

In addition to what latexman rightly wrote about the problem of routing the PSV discharge to the pump's suction line, i think two major reasons for which the PSV discharge should be conducted to the relevant suction drum are as follows:

 

1-Hydraulic resistance to receive the incoming fluid through the PSV discharge LINE by suction LINE while there wouldn't be such resistance in the suction drum.

2-Pulsating behavior of the PD pump will lead to sucking no fluid by suction line during half cycle of forwarding the fluid within which the check valve for sucking the fluid from the PSV discharge line is closed. Then the PSV discharge fluid will face higher back pressure and if can overcome to this pressure has to go to the suction drum rather than toward the pump.

[shvet1]

I should say that I have ~10 years experience in operation of reciprocating and membrane pumps and I have not encountered or have heard from collegues of issues related to such PSV design. This design is ok unless one uses this PSV as a sort of a control valve during routine operations.

As always details are crucial

Edited by shvet1, 24 July 2024 - 11:36 PM.

[breizh]

Hi,

Probably good to read:

https://www.cheresou...lacement-pumps/

Breizh

[QuantumEng]

If depends. What’s the fluid? And, how stable is it, if heated via recirculating? And, how long will it take the pump recirculating to raise the fluid temperature to a unsafe temperature?

 

latexman,

 

The fluid is water initially at 32 C and about 1 barg. If the PSV is set to 10-15% above to normal discharge pressure of 4.6 barg, I don't foresee heating from recirculation into the PD suction side an issue. If there were issues from the installation, it may be backpressure related as fallah mentions. 

[shvet1]

Note that routing PSV discharge back to a liquid source may have to do with a small DN long stagnant piping, often sagging. Design showed in the extract from ANSI/HI 6 under the link above is suitable for cases when the pump is located nearby to the tank and the pump has a start-up line that can be combined with PSV discharge.

 

 

There are many cases where there is no a one source where a liquid is pumped out, e.g. tank farm, or there are no other lines are able to be combined with PSV. I personally have the bad experiences with long liquid-filled stagnant piping under pressure and caution others from such design.

PSV design has no a simple answer like "do [this way] and all will be ok". All factors should be evaluated and the best option validated.

 

Note that PSV design may differ from conventional spring loaded, e.g. unaffected by backpressure

ANSI/HI 6.1-6.5-2000

6.3.7.1 Relief valve

The insertion of a discharge relief valve of suitable size for the rate of flow of the pump, set to open at a pressure above the operating discharge pressure required of the pump, is mandatory because of the safety it affords. The relief valve should be placed in the discharge line close to the pump and ahead of any other valves.

Full-opening, pilot-operated, or shear pin relief valves, or burst-discs, which require little ot no over-pressure to develop fully-open flow capacity, are often preferable to spring-loaded relief valves. Spring types may require considerable over-pressure to compress the spring before becoming fully open, creating pump overload.

...

 

Also note that a soaked spring may be prone to a failure on demand. I have not met clues that an extensive industry study was conducted to assure that conventional spring-loaded valves preformance is not affected by liquid in the discharge chamber - may be someone can share such?

CCPS Pressure Relief and Effluent Handling Systems 1998

2.11.1 Relief Device Reliability

...

One analysis of data of the first type shows that about 44% of the valves lifted outside the limit of +/-10% of the original set pressure, regardless of the time in service (one week to one year for these data) (Aird 1982). This limit is the criterion used by one company to define failure for the purpose of establishing maintenance schedules. The “failed” valves were apparently quite clean; fouled valves could not be tested before cleaning. Initial relaxation of the spring is one possible cause of low set pressures. This 5 to 10% reduction can be avoided by compressing the spring almost all the way for a few minutes before installation in the valve. Subjecting a valve to vibration was shown to cause a progressive reduction in set pressure in a laboratory test. Another valve was soaked at 80°C, causing a 75% increase in set pressure over a period of 800 hours. Both valves  returned to the original set pressure after the initial test lifts. The observed wide deviations from the original set pressure remain unexplained.

Edited by shvet1, 26 July 2024 - 02:25 AM.

[astro]

Hello all,

 

I have seen various posts regarding the location of discharge for PSVs protecting piping/systems downstream of a PD pump for a blocked outlet scenario. Notable contributors such as Art Montemayor and fallah have stated that the discharge piping of the PSV should NOT be routed to the suction side of the PD pump.

Is there an explanation you guys can provide as to why this is not acceptable? Perhaps any documentation/source that supports this. 

I am in agreement with not routing the discharge to the suction side from a logical standpoint, but is there any further engineering evaluation behind it?

Many thanks in advance.

The concern with recycling PSV discharge to the suction is typically temperature rise.

 

GPSA 14th Ed (SI) provides a simple method to estimate this behaviour. See pic. Nomenclature and units as follows:

• H = total equipment head, m of fluid
• e = pump efficiency, fraction
• C_p = specific heat at average temperature, J/(kg • °C)
• t_r = temperature rise, °C

You could do a simple time slice, step wise calculation based on the mass of liquid being recycled and get a feel for time frame. All get's a bit complicated when also considering system heat loss but at least you can have some fun working up the numbers.

Attached Files

•  Temperature Rise.jpg   32.87KB   1 downloads

Edited by astro, 02 August 2024 - 10:18 AM.

[breizh]

Hi,

What others said:

https://www.michael-...allation-advice

 

https://up.codes/vie...acement-pumps#8

 

Breizh