PSV requirements on Pumps

[kdt]

Is there a section of the code which deals with the requirement for PSVs on Pumps for Thermal Expansion case?

I am used to seeing on heat exchangers and vessels which can be isolated but not pumps.


Another side question on the same topic. Generally when are PSVs required on piping for Thermal protection? Again, is there a section of code that deals with this issue? Typically I have not seen this inside plant buildings, just long pipes that are above ground containing gas/hydrocarbons and exposed to solar radiation.

 

[Montemayor]

kdt:

Pumps are not normally blocked-off at the suction, causing the contained liquid to be constrained. To do this, in many cases, would cause a loss of prime and would be a pain in the backside. That's why most pumps are kept flooded with the suction fluid - ready to be started up again when needed.

That doesn't mean you can't block off a pump and install a thermal expansion device. I would certainly do this if forced to block off the pump and seal in liquid subject to expansion by external heat (like a fire?).

I don't know which "code" you refer to; ASME nor the Hydraulic Institute have such a code. Heat exchangers can easily be blocked off on one side while maintaining a heat source on the other side - that's why a thermal device or PSV is always considered. This is normal practice. It is not normal practice to have a pump operating in an atmosphere that can supply sufficient heat to a trapped fluid to cause it to expand - except in instances like cryogenic pumps or liquefied gases. I've dealt with many of these through the years and always made it a point not to be able to trap the liquid in the pump.

I hope this addresses your concern.

 

[MJCronin]

kdt,

In addition to te comments made above, you should also consider that relief of liquid pressure through the pump seal would be the most common result of a "blocked in" pump.

Some high energy pumps, in a blocked configuration, have been known to cause the liquid to flash and rupture the pump case.

"Seal-less" pumps (also known as canned or mag-drive pumps)are another special case.

There is an API recommended practice (APR-RP520) that provides guidance and suggestions on calculation of thermal relief devices for blocked piping systems. You must know the "cubical expansion coefficent",specific gravity and specific heat of the fluid of interest. You must also estimate a heat influx rate. Typically, the smallest pressure relief device can protect a very long liquid filled line.

My thoughts only.....


MJC

 

[kdt]

Montemayor,
I agree that pumps are not normally blocked-off at the suction. However, many applications have isolation valves on both the suction and discharge to allow for maintenance of the pump in instances where pumps are installed in parallel, or to prevent draining the entire system to allow for work to be performed.

Although these valves are not normally closed, they are installed and therefore there is a potential for the pump to be blocked in (granted the isolation procedure should specify that the vent/drain valves be opened).

Say the fluid being pumped is water at 4°C. The pump house in summer has a HVAC temp of 30ºC. There is a change in density, granted a small one, but as water is incompressible there would be a substantial increase in pressure.
Are thermal PSVs required? or am I overanalyzing the situation?

As for 'code', I was thinking of ASME and also provincial/state legislation/acts regarding pressure piping and vessels.

 

[Montemayor]

kdt:
As I stated before, there are no known "codes" that mandate a thermal relief device. Additionally, I also stated that if I have to do it from a safety concern, then I have to do it. There are "recommended practices" - API 520 & 521 (parts I&II) that go into this. You can do it or opt not to do it.

A 1/2" thermal relief valve (approx. $150) will protect the largest pump you can come up with. There are no calculations required to "size" or specify a thermal relief device. Even OSHA does not require engineering calculations for these devices. You simply buy one and install it. The amount of expansion fluid is so small, it isn't worth anyone's while to go into an academic exercise to determine the exact amount.

 

[roker]

hello,

1.it is very unusual to protect centrifugal pumps with thermal psv's, in big pumps you install a minimum flow line from the discharge to protect the pump from overheating in case the discharge valve is closed.
also the centrifugal pump is designed to higher temp and pressures than the working conditions.
you can also install a locking device on the suction valve so you close the suction and discharge valves only for taking out the pump from service.
it is also recommended that the suction line from the valve to the pump shall be in same piping spec as the discharge.
2. in positive displacement pumps you have a psv internal or external installed to circulate the fluid pumped from the discharge back to the suction but here the reason is to protect the pump from overpressure if the dischage is blocked since a positive displacement pump can develop very high pressures beyond the design pressure of the pump.
3. see API 520 and API 521.

regards

 

[svanels]

I have seen pumps with external thermal relief valves, but that is on pumps with external cooling.

 

[ScottPearson]

I've seen Relief Valves installed on Transfer lines that run from outside tank farms to inside buildings. This is usually for Thermal Expansion since the fluid may see extreme temperatures inside and outside (in the winter with hot rooms inside). The PSV's are usually small and installed at either end of the line. I concur that the Codes do not require them. If the pipe runs are long or the fluids within could be considered hazardous in the event of a release, the relief valves make a little more sense than normal duty/service.

Trusting the internal Relief on Positive Displacement pumps is a usual no-no in my experience, mainly because the pump manufacturers cannot provide the required calculations for relief capacity, plus having that additional external relief is a good psychological and physical reminder of the safety needs of the system.

 

[sshep]

With respect to having spare centrifugal pumps in a standby line-up, my experience has been to have the suction isolation valve closed, and leave the discharge valve open.

This seems to me to be a good line-up practice because of the check valve which is typical on the discharge of most pumps- an open discharge valve keeps from trapping an isolated pocket liquid in front of that check valve. I have seen thermal expansion due to heat transfer through a closed discharge block valve cause a leak at that point (i.e. one such block discharge overpressure failure we encountered on a standby hot oil pump was potentially a serious leak). On the otherhand, the blocked in suction valve keeps any check valve leakage from recirculating around the operating pump- in extreme leakage situations you can even counter-rotate the standby pump which can cause damage.

This is just my experience, others may have different experience. Best wishes- sshep

 

[dcasto]

I specify 3/8" tubing check valves around either the suction or discharge valve that could cause the blocked in system as a way of handling thermal relief.

 

[reena1957]

Dear kdt,
I did a calculation for yr case and got 100 atmospheres pressure rise. If you have slightest doubt that the pump will be blocked in, put in a TSV. I have seen a blocked in oofsite pump blow to pieces out to a distance of 200 meters, fortunately on a Sunday with no grevious consequences!! The vent was plugged and drain valve wheel did not catch and was slipping. The maintenanace technician was called to attend the drain valve who got delayed sufficiently to get the blocked in pump heated up and blow away to pieces.