PSV for Positive Displacement pump 1

[C4Reactor]

All,
I need you opinion on this one. I'm HAZOPing a PD pump set-up. For the pump block-in condition (discharge valve close), is it sufficient to depend on the pump's internal relief valve or must we install a PSV upstream of the discharge valve with its outlet being recirculated into the suction line.
Appreciate any comments.
Thanks

 

[TD2K]

Lots of companies that I've seen require an external PSV rather than relying on the pump's internal PSV. The rationale is that you can tag it, make sure it's on a regular schedule and it's easier to tell if it's passing.

 

[psafety]

C4Reactor,
It truly depends on pump manufacturer. The integral reliefs used by Waukesha are NOT designed for full flow bypass. On the other hand I've seen others such as Viking pass full-flow.

Keep in mind; we in process safety assume all energy is input to the fluid in this condition and volitility of the material must be considered.

psafety

 

[EGT01]

Good points by TD2K and psafety.

My experience is the same as TD2K. Another point I've heard about the pump internal PSV is that it typically must be on the pump to properly set and makes routine testing more difficult.

In one case I've seen where the pumped material had a high melting point and hazardous sensitivity to temperature (detonation potential), a high integrity pressure protection system (high pressure trip) was used instead of an external PSV. This avoided the concerns of heat tracing on stagnant material in the PSV piping.

In another case where the PD pump was a diaphragm type with the diaphragm driven by another fluid which had a relief valve, there was discussion about taking credit for that but I'm not sure the outcome. Again, if its not a PSV you can take off and easily maintain, it would seem you are back to not counting on it.

Not that I disagree to psafety on the energy input issue, I've see photos of what happens when high horsepower pumps get blocked-in. But that has got me thinking, is there any concern for the internal PSV that comes with a PD pump?

 

[psafety]

Maintaining either; an integral or external valve is a must. Preference is to provide "on-line" testing capability (assume we are discussing protection of the pump, and NOT downstream equipment). I try to assure there is a block valve in close proximity donstream of the pump and a PI (pressure indicator) between. We test reliefs yearly and maintain history.

psafety

 

[pmureiko]

You need an internal or external PSV on the pump discharge. The internal PSV is considered acceptable is the flowrate is low and the motor is small, say less than 50 gpm and 5 hp respectively. Basically the risk is considered low enough to rely on a less reliable internal PSV.

 

[CHD01]

Everyones advice is good advice. This is an area where protection practices vary quite a bit between industries. Basically it is difficult to maintain and verify internal relief valves and normally we try to provide a separate external relief device (a device on the non-process side of a diaphram type pd pump may also acceptatble) where: the process is flammable, or hazardous, or the pump systems failure will result in significant (you have to define what significant is) damage or loss of life. Fractional hp pd pumps may therefore not need an external relief device since significant damage MAY not occur.

One note that escapes many is that depending on the type of PD pump being used, the discharge rate may be a function of Pi - which means that many people may undersize the relief device by a factor of at least 3.1416.

The more you learn, the less you are certain of.

 

[CHD01]

I wanted to expand on my comment regarding PD pump discharge rates being a factor of Pi:

This situation occurs for reciprocating type pumps; for SIMPLEX pumps the required relief capacity needs to be based on the maximum instantaneous rate and therefore the maximum flow rate should be multiplied by Pi. This is because the flow rate varies with the stroke of the reciprocating pump. Thus the max instantaneous rate is equal to the published rate x Pi. While I have not encounted DUPLEX or TRIPLEX pump applications, I know that one should contact the vendor for instantaneous flow of these pumps.

The more you learn, the less you are certain of.

 

[EGT01]

CHD01,

I'm still a little unclear about the basis for multiplying the max flow x 3.14. Is there a reference you could suggest that I could study further?

 

[Cormoran]

Dear folks,
In case of external PSV, flow return to suction of the pump or to storage tank?
Alfredo

 

[CHD01]

Cormoran: Better to go to storage tank assuming it is close by. Going to the pump suction can be a problem depending on tie-in points, how system is valved and process operation.



The more you learn, the less you are certain of.

 

[CHD01]

EGT01:

A single plunger controlled volume pump delivers liquid in a pulsating flow with the rate of delivery following a sine curve. For multiplexing pumps the flow tends to be smoothed out. When a duplex pump is used, the drivers for the two liquid ends are spaced 180 degree apart on rotation so that one pump is on the suction end when the other is on the discharge end. Normally you have pulsating flow (you can install accumulators to get pulse free flow if required) but this is not always a concern since the pump is delivering a constant volume of liquid per STROKE! But if you have a closd discharge, you want the maximum instantaneous flow which is a function of the stroke - or a sine function - or a function of pi.

Remember too regarding rated capacities that the flow capacity delivered is a function of the stroke length, the number of strokes per minute and the piston diameter. It is best to check with manufacturer to determine what the instantaneous rate actually is. But, be careful what we ask of the manufacturer since it is easy to confuse max pump capacity with max instantaneous rate.

The more you learn, the less you are certain of.

 

[CHD01]

EGT01: I would think vairous manufacturere's would have info on web - try a search for MILTON ROY reciprocating pumps or others.

The more you learn, the less you are certain of.