13 replies to this topic

[CSNK]

Dear All,

I have one doubt regarding relief requirement at the discharge of Positive displacement pump (rotary).

1. I have a rotary pump with casing design pressure at 16.0 Kg/cm2g, discharge piping design pressure at 13.0.
2. PSV is provided at the discharge piping to protect the pump and piping in case of inadvertent closure of discharge valve.
3. Also rated discharge pressure is 9.22 Kg/cm2a.

My doubt is that set pressure of PSV is given as 10.26 Kg/cm2g. - why so?
Why this set pressure is taken when casing and piping design pressure are quite higher than this.

Actually i am doing adequacy check for this PSV.
Thanks.

Edited by chiks, 19 July 2012 - 07:06 AM.

[Robert Montoya]

Dear; For any pump requiring a PR valve for its protection or for protection of downstream equipment, the PR valve set pressure should be higher than the normal pump discharge pressure by 170 kPa or 10% of the set pressure, whichever is greater. Note, however, that in some cases a higher PR valve set pressure may be desirable to assure a sufficient differential when the pump is to be operated under lower than normal design pumping rate. This will recognize the higher pump discharge pressure under low flow conditions. In the case of reciprocating pumps, a greater differential than 10 % is desirable due to pressure surges.

So 9.22Kg/cm2+1.734(170kPa)= 10,95 kg/cm2

Edited by roberdani12, 19 July 2012 - 08:35 PM.

[Lowflo]

<sub>Chiks -

check the motor capability. It's very possible that you don't have enough motor to get to a higher pressure. The PSV might have been set at 10.26 to prevent the motor from overloading.</sub>

[Art Montemayor]

If you are “checking” the adequacy of the PSV, do you have the backup calculations that justify the capacity and pressure setting for this PSV? If not, have you generated these required calculations? This documentation – like all PSV calculations and documentation – should be filed in the plant’s engineering documentation record. Otherwise, no one knows the background or the justification for the sizing and set pressure.

You could have a variety of reasons for the set pressure being what it is. For example:

• The pump casing MAWP (or “design” pressure) doesn’t mean that it should be the base for deciding on the set pressure. The casing may not be the weakest component in the pump assembly. It could be the pump’s shaft seal.
• The piping design pressure never is taken as a guide for determining relief on the piping system. In almost all cases, it is the flanged connections that are the weakest components – not the piping itself.
• As Lowflo also notes, it could be that the setting is meant to protect the electric motor driver.

What I am trying to emphasis by the above points is that plant engineering records should, or must, exist to confirm and prove the correct and accepted protection level for the pump’s system. If they don’t exist and you are generating them, then moreso reason to be aware that there are a variety of factors that must be taken into consideration before arriving at a set PSV pressure that will ensure safe operation for both humans and machinery.

As an example of the above logic, I can vouch for DuPont’s approach on protecting rotary gear positive displacement pumps. DuPont never accepted the pump manufacturer’s standard PSV on such pumps without a fully documented set of calculations confirming the capacity and pressure setting for the pump. It was my personal experience in such applications that no major pump manufacturer was able to satisfy DuPont’s requirements on this issue and DuPont project engineers had to do the calculations and specify a proper and documented PSV that was installed. The manufacturer’s PSV was never relied upon to protect the installation. I always removed the manufacturer's PSV that normally discharged fluid back to the suction side of the pump and plugged up the opening. I relied on the installation's calculated and documented PSV as furnished by us - and I never discharged to atmosphere or the suction side. We piped the relief discharge to the source tank.

This was true during the 1990’s and I am sure that DuPont still takes that stance today. Other major industrial firms in the USA also sponsor this type of policy. The specific capacity and pressure setting of these PSVs has to be calculated and documented in engineering files.

[CSNK]

First of all thanks to all for providing valuable feedback.

Dear Roberdani12,
As per your calculation Set pressure of PSV comes out to be 9.94 Kg/cm2g. (since 9.22 is in absolute).
Can i get reference of this regarding 10% or 170 Kpa. whichever is greater to be used?

Dear ART,
You are absolutely right that there are no previous records available.

Dear Lowflow,
Regarding motor i dont have much details at present, i just know the power i.e - 11 Kw rated and 7.05 at the operating pressure.

[Robert Montoya]

chiks:

Dear this a criteria is used by different companies and is related with the maximum allowable working pressure MAWP. I think the approach is that the set point is equal to the MAWP.

But once it is verified that the vendor to deliver data to the pump to verify that the design pressure of the equipment is below the set point of PSV

Is used by Fluor Daniel (Process design manual), and appears in:
- ludwig - applied process design for chemical and petrochemical plants, volume 1
- Pressure Safety Design Practices for Refinery and Chemical Operations. vol 8

Robert

[pallavikhatri]

I have one more query related to the PSV for positive displacement pump. Is it necessary to provide the installed spare PSV for the pump?

 

Pallavi

[fallah]

Pallavi,

 

It depends on sparing philosophy of the relevant project. But as far as i know and have ever seen, there is no need to consider installed spare for TSVs and the PSVs for protection of PD pumps.

[pallavikhatri]

Thanks Fallah for the reply.

But what is reason for not providing the spare PSVs for the PD pumps and the TSVs

 

Pallavi

[fallah]

Pallavi,

 

There is no especial requirement in codes and standards such as API and ASME to show sparing is mandatory for a PSV. Normally, plant owner might made decision to consider PSV spare in order to provide higher reliability and would implement this requirement in relevant design specifications to be followed by engineering contractor.

[chsnoopie]

If a PSV is installed at the discharge line of the rotary pump,with the downstream of PSV back to the pump suction,may i know how to determine the backpressure of the PSV?
Should it be the suction pressure of the pump?

[Art Montemayor]

Never pipe the discharge of a relief valve installed on the discharge of a pump back to its suction.  This is asking for trouble.

 

Always pipe the PSV's discharge all the way back to the source  or tank where the fluid originated.  This has been repeated many times through the years and I would have thought that this was well-known.  The money "saved" in piping directly back to the pump suction line is pennies compared to the possible damage and operating problems you can generate.

[fallah]

Hi,

 

At first it is better for you to follow Art recommendations to configure the PSV tail pipe termination for PD pumps...

 

However, if you want to know how to calculate the PSV back pressure if its tail pipe terminated to the pump suction line:

 

The superimposed back pressure would be in variation between max/min pump's suction pressure...

The build up back pressure value depends on the tail pipe route/size and also PSV relief load...

[chsnoopie]

I see. Thank you very much for the information. =)