Pressure Rating of Parallel Pumps in Series-Revamp

[Sawsan311]

Dear All,

Considering brownfield revamp of existing 3 x 50%parallel pumps which require to have higher discharge pressure. Hence,if an Engineer proposes to install another 3 x 50%in series with the existing pumps such that we have two pumping systems in parallel for each set but in series among both sets.

The new parallel set of pump would be installed downstream the existing parallel pumps, I have a question whether the existing pump's casing design pressure shall be ensured to be within the combined system's shut off pressure since a common mode power failure scenario may cause a blocked outlet scenario downstream the 2nd set of pumps with a failure (and no credit) to be considered for the minimum flow control system provided at the individual pumps.

Do you agree that the overall system's pressure rating starting from the suction isolation valves upstream the first set of pump to be suitable to withstand the shut off pressure calculated based on the (2nd set discharge pressure-1st set suction pressure)+ suction system's design pressure.

Thank you very much,

Regards,

 

[georgeverghese]

Yes, since reverse flow through the check valves on each of the stopped pumps of a series set, with the stage 1 suction block valve closed, could result in the first stage pump experiencing 2nd stage pump discharge dead head pressure.

 

[LittleInch]

As George says this all depends on whether any fluid coming back has anywhere to go or not.

Sometimes a relief valve will allow return flow whist the upstream esd valve closes if there are valves upstream the first pumps which could close and therfore maintain the lower pressure rating.

You need to HAZOP this carefully when you have a design.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

One way to mitigate the risk of this overpressure scenario on the 1stage pumps would be to enable both of the following:
a) Install 2 check valves on the 2nd stage pump discharge, and have at least one of these inspected every year as part of PM
b) Install an RV (with setpoint at stage 1 pump MAWP) in parallel with the min flow control valve on the stage 1 pump discharge, and size the RV for say 5% of the max forward flow on this combined pump set.
See if this passes scrutiny at a mini HAZOP.

 

[EdStainless]

Draw it out, look at flows and pressures across the entire operating range.
Consider what happens when a pump fails, or a check valve fails.
Do a HAZOP, and be careful.
Systems like this can work fine, but there are a lot of ways to get it wrong.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Sawsan311]

Thank you all for your feedback,

For Mr.LittleInch (Petroleum), I believe your response is same as the response of MR.georgeverghese (Chemical). Thank you,

Dear MR.georgeverghese (Chemical), would it be possible to install high integrity pressure trip system which trips the complete pumping system when the discharge pressure experienced by the 1st set of pumps (lower pressure pumps) approaches its limitation in terms of existing casing design pressure and pump's shut off pressure. The subject Safety function can be evaluated to ensure having a SIL rating of level 3 as explained in API 521 to have credit for favorable instrumentation response in place of providing a pressure relief device.
Thanks

 

[georgeverghese]

The integrity level of the SIL system may be reduced by adding one or more of the other safety devices ( higher integrity check valve, RV with relief capacity or similar). Relying purely on IPS is not a good idea and may not pass a SIL review. In fact, without an RV, this IPS may need to be SIL 4, which is too cumbersome / too risky.