Piping Pressure when Pump is Dead-Headed
Piping Pressure when Pump is Dead-Headed
(OP)
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
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





RE: Piping Pressure when Pump is Dead-Headed
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.
RE: Piping Pressure when Pump is Dead-Headed
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.
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Also: There's usually a good reason why everyone does it that way
RE: Piping Pressure when Pump is Dead-Headed
RE: Piping Pressure when Pump is Dead-Headed
RE: Piping Pressure when Pump is Dead-Headed
RE: Piping Pressure when Pump is Dead-Headed
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Piping Pressure when Pump is Dead-Headed
Thanks
RE: Piping Pressure when Pump is Dead-Headed
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
RE: Piping Pressure when Pump is Dead-Headed
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.
RE: Piping Pressure when Pump is Dead-Headed
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.
RE: Piping Pressure when Pump is Dead-Headed
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.
RE: Piping Pressure when Pump is Dead-Headed
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
RE: Piping Pressure when Pump is Dead-Headed
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.
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RE: Piping Pressure when Pump is Dead-Headed
RE: Piping Pressure when Pump is Dead-Headed
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
RE: Piping Pressure when Pump is Dead-Headed
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.
RE: Piping Pressure when Pump is Dead-Headed
RE: Piping Pressure when Pump is Dead-Headed
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...