Piping Pressure when Pump is Dead-Headed

[BanditKeith]

Hello Everyone,

I've been asked to check if some existing piping needs to be re-rated based on the maximum dead-head pressure exceeding the design pressure of the suction and discharge piping on a few pumps. I got a few questions before I dig into this in more detail:

1. When a centrifugal pump is dead-headed, should I be concerned if the dead-head pressure only exceeds the design pressure of the discharge piping, or should I be concerned with the suction piping as well? I think it should only matter on the discharge piping since pumps produce a differential head, and the energy from the pump is essentially being delivered at the discharge, correct?

2. If I want to determine the pressure the piping can experience at dead-head conditions, will it be equal to just the pump dead-head pressure, or should it be equal to around:

(Pump Dead-Head Pressure) + (Static Pressure from Tank/Vessel/Whatever) + (Hydraulic Head)*(Weight of Fluid) - Frictional Losses

It's been awhile since I've dealt with centrifugal pumps, so I apologize if this is a no-brainer for y'all.

Thanks

 

[quark]

It's not a no brainer, at least for me, but strange because we use pumps for pumping and not for not pumping ;-)

When the pump runs at shut off condition, frictional losses vanish as there is no flow. Pressure in the system is due to water column and gradually increases towards pump discharge.

 

[LittleInch]

As ever it depends on your system, how it is operated and where different valves are. If e.g., there is an NRV on the discharge side then it is unlikely that any dead head pressure would make it back to the inlet pipework. On the other hand if there is a long downstream piping section which is pressurised and then you can isolate the pump on the inlet side, then it could see back pressure more or less equal to the dead head pressure on the inlet side of the pump. Only you know your system / P & ID / mode of operation. It is not unknown for the inlet pipework back to the isolation valve and entire pump and discharge piping being set to the same design pressure.

The max discharge head would comprise maximum head at no flow from your tank or whatever plus your no flow differential head of the pump. As there is no flow there are no friction losses. Work in head then convert the final number to pressure.

Pressure (bar) = (m X Sg) / G.

There is a similar equation for psi - there are many conversion websites available.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[TenPenny]

At dead head, the pressure at the discharge of the pump will be pump dead head + whatever suction pressure is at the inlet of the pump (ie, tank pressure, elevation, etc). That's why most pump suppliers want to know what you MAXIMUM suction pressure could be at the pump inlet, so they can determine the maximum dead head outlet pressure at the pump. The pump itself has to be able to handle this, too, don't forget.

 

[SNORGY]

Caution on the addition of PRESSURES on both sides of the pump - it's the HEAD that is added. The discharge PRESSURE on the outlet of the pump is what it is.

 

[QualityTime]

The maximum pressure you will ever see on the piping is the shut in head on the pump curve

 

[LittleInch]

Quality time. No, that figure is the maximum differential head. If your head is zero m then it's correct, but normally it's more than zero and needs to be added to the inlet head, which can often be quite large.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[rotw]

Can someone please give a reference to a clause from an industrial standard/code (API, ASME, hydraulic institute, or whatsoever) that eventually covers this issue especially with regard to a (possible) safety margin requirement on the design pressure that applies on top of the max discharge pressure ?
Thanks

 

[LittleInch]

There isn't one, as far as I know. You can have the design pressure equal to the max discharge pressure.

However, in practice, this makes it very difficult to set suitable trip settings or settings for things like relief valves. For piping the tendency is therefore to go for at least 5%, providing it doesn't take you into the next pressure class, but is dependent on your particular circumstances and the accuracy of your instruments and relief valves, if you have any.

Piping codes allow a certain amount of time per year in operation operating above the design pressure so if the excursion is relatively rare then it may not be a problem.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[1gibson]

Rotaryw, 6.3.5 in API 610 11th defines MAWP (for pumps that do not have a specified minimum already, 40 bar for most types) as maximum discharge pressure plus 10% of the maximum differential pressure.

The +10% is for positive test tolerance on head, and 5% future head increase which is a requirement for pump selection (6.1.4.)

6.3.1 defines MAWP as max suction plus max differential for normal SG, rated impeller diameter, and rated speed
6.3.2 (bulleted, must be specified by purchaser to be applicable) allows purchaser to request "max discharge pressure" to consider max SG, max diameter impellers/# of stages, and/or operation at trip speed.

In reality, it is good practice to consider the max SG even if not specified. Take or leave the max diameter/#stages, depends on the details, usually only when specified. If customer wants to include provisions in the original design for adding a stage in the future, of course it is prudent to consider that case for the MAWP. And I would hope nobody lets a pump overspeed at closed valve, certainly not the pump vendor's problem if it happens, so only consider that when specified.

 

[rotw]

1gibson,

Should we align the piping design pressure to the pump casing MAWP, is this a common practice ?

What if there is a pressure safety valve in the system (at pump discharge) ? My guess is that this would allow to relax the MAWP but then the piping should be designed according to PSV set pressure which would be allowed to be lower that the max. pressure resulting from current shut off head.

By the way, I think LittleInch's response refers to whether a guideline exist for setting piping design pressure based on pump shut-off head which is also what I meant. API610 has a requirement on MAWP but that is for pump casing design pressure.

 

[1gibson]

I would think so, but I don't do piping. I don't have any references from piping codes.

Pump MAWP considers all the realistic factors, so it seems silly to start with the pump's shutoff head when you could already have the answer with pump MAWP. But I realize that was not the question, so I think my comments here are complete. Interested to see some more responses that are closer to the mark than mine.

 

[LittleInch]

Aligning the piping design pressure to the MAWP of the pump is a good start point, but not the whole answer. Piping design needs to allow for the worst case in NORMAL operation (301.2.1 in ASME B 31.3). If there are either control valves, pressure limiting switches or devices or the overpressure only happens occasionally, this can act to reduce the piping design pressure. Sometimes this is important, sometimes not, but if it takes the piping into a different flange class rating , .e.g. 300 to 600, then costs can increase dramatically for something that is not required.

As the MAWP of the pump is only likely to occur at dead head conditions, you need to understand your system to see how often this can occur to see whether this is classified as normal operation or can be regarded as occasional overpressure, which for 31.3 can be in the region of 20% over design pressure.

I realise this isn't a hard rule, but there are so many combinations of circumstances, each one needs to be considered on its own to come to the correct design pressure.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

I wouldn't think that it is especially good practice to design your piping for less than shutoff head, i.e. put your shutoff head pressure into the "overpressure" range of your piping. Shutoff pressure happens every time you turn your pump on, or off. Good practice would imply that any system should be designed to allow a stable start-up or shutdown without any excursion into the overpressure range. Shouldn't "normal" operations include normal starts and stops. Would an aircraft be designed so that it had to land at speeds above it's maximum red-line speed? Of course not, but it does illustrate the problem. An aircraft might be designed so that it could be landed above red-line speed, but practice would only allow that in a dire emergency.

When I've had similar situations before, I have designed the pipe immediately adjacent to the pump discharge for shutoff pressure and installed a pressure control valve and relief valve after that with set pressure below the design pressure of the remaining piping downstream.

I hate Windowz 8!!!!

 

[racookpe1978]

In all of the above, remember the pump seals need to be rated at that maximum pressure as well as the inlet and discharge piping.

 

[LittleInch]

BI. I take the points and in trying to be brief missed out the bit about the piping from the pump to the isolation valve sometimes needing to be a higher rating than the rest of the pipework to cope with start/stop operations.

There are many ways of stopping a pump which doesn't result in shutoff pressure being experienced every time. I did say that each system is different and what is classified as "normal" operation will vary.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[moltenmetal]

Consider the following situation: a centrifugal pump has a perfect check valve on the suction and a closed valve at the discharge, with the whole system filled with liquid perfectly. When the pump is started, it delivers the suction pressure plus the dead head TDH to the discharge piping- frictional losses are zero because there's no flow. When the pump is stopped, because the fluid is incompressible, that total discharge pressure should now be transmitted to the suction piping as far as the check valve, should it not? That will not persist long in reality as the check valve will leak (as they all do!), and won't have to leak much for that pressure to fall to the suction pressure, but the pressure does exist for a time after shutdown from a dead-head situation.

Similarly, in a PD pump, leakage through the suction and discharge check valves will over time, transmit discharge pressure to the 1st closed block valve on the suction piping, assuming that the discharge block valve is left open when the pump is shut down.

It's not our normal practice to do so, but you could make an argument for designing suction piping up to and including the 1st piping block or check valve for the discharge rather than the suction pressure.

As to selecting a prudent design pressure for the discharge piping, that's determined by the type of relief device you intend to use to protect the discharge piping. Since the MAWP of the piping must at least equal the relief pressure, and the relief pressure must have the relief valve comfortably seated (i.e. not bouncing or simmering) at the maximum operating pressure, there must be a margin between the max operating and relief device setting. That margin depends on the type of the relief device selected, and whether or not the flow is pulsile. If there's only an internal pump protection relief device, generally you are not permitted to rely on that for protection of your piping because it doesn't handle thermal expansion or other blocked in relief cases.

 

[SeanB]

No you don't have to align the piping design pressure with the casing MAWP. I usually set the downstream piping design pressure as the maximum discharge pressure - which is shut off head plus maximum suction pressure. However, keep in mind that if you ever put a bigger impeller in the pump your maximum discharge pressure will increase - so now you are above the design pressure or the piping. So....this is why it is good practice to make sure that the piping is hydrotested to the maximum allowable for the line class.

 

[rotw]

quoted
However, keep in mind that if you ever put a bigger impeller in the pump your maximum discharge pressure will increase
unquoted

yes it will increase but in no way the pressure shall exceed the pump casing MAWP.
I have the feeling we are running in circle...