PD Pump to Heat Exchanger, where should Relief valve go?

[andrewfreeman]

Greetings,

I'm designing a test rig for our R&D test facilities. Part of the rig requires water at 40barg close to saturation point.

Therefore we have a PD pump raising mains water to 50barg going through a heat exchanger (rated to 60barg) which uses steam as the heating medium. Max flowrate will be 300kg/h of water.

We we told it was optional whether we wanted to fit a safety valve on the outlet of the heat exchanger (in case the plates rupture inside, leaking steam into the water, thus increasing the water pressure).

However I have just discovered I will definitely need a safety valve after the PD pump incase of a blockage in the line.

My question is, would it be acceptable to have just one safety valve located after the heat exchanger that serves both the PD pump and heat exchanger? I understand that the safety valve should be fitted as close to the pump as possible, but there will be nothing inbetween the pump and heat exchanger, and we will place them a close together as possible.

I know safety is paramount, but I don't want to go overboard. Most of the time we will be operating at much lower pressures (around 20barg), but on rare occasions we will go to the pressure extremes of our rig.

Thanks

Andy

 

[BigInch]

I'd put a safety valve set for 50 Barg in the line between pump and the xchgr and I believe that, if the water line is rated at lower pressure than the pump, there should be another located on the water line, if there is any possibility that you could rupture a plate in the exchanger. 50 Barg of steam and water is not something you want to take any chances with.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[quark]

I fully agree with BigInch. Pump discharge is the first choice. Anyhow, you have to have the discharge piping rated to atleast 40 barg, so why not make it 50 barg?

 

[itdepends]

I'd put a safety valve on the discharge of the pump- prior to any valves in the line (so you can't isolate the pump from the safety valve). If there are no isolation valves between the pump and the HX- then this PSV would also protect you from overheating the water.

Interesting comments RE plate rupture- I assume the steam you are using is less than 60bar- so even if you did rupture a plate- you wouldn't overpressurise the HX (it would make a racket though). I always thought the risk with HX's was blocking them in on the suction/discharge of the cold fluid with the failure being caused by thermal expansion of the cold fluid?

 

[andrewfreeman]

thanks for the advice everyone,

The max steam pressure available for the heat exchanger is 40barg anyway, so your right even if a plate does rupture the heat exchanger will be safe.

My only concern is that we planned to have ANSI 300 iso valves and check valves downstream of the heat exchanger. The water we are heating up and pressurising is to be injected into a steam line. The max pressure for this is 40barg so we have opted for ANSI 300 flanges and valves etc on the main steam line. As we've got a piston pump, it will pump to the pressure required to get the water into the steam line. So in theory the water pressure before injection will just be slightly higher than the steam pressure.

I thought if I put the safety valve downstream of the heat exchanger and set it for 41.6barg (the max pressure for ANDI 300 for sat. steam), it would protect the components downstream (which I really need to keep at ANSI 300 for cost and weight issues), protect the heat exchanger and protect the pump.

There will be some pressure loss through the heat exchanger and piping system, so I estimate (from past experience) that if i wanted water at 40barg out of the heat exchanger then my pump will be pumping to 45-50barg. Therefore if I have the safety valve after the pump, then I would have to set it higher than what the limits of my components downstream of the heat exchanger could handle.

Sorry for the long winded replies, just want to describe my situtation accurately.

Thanks

Andy

 

[codeeng]

Piston pumps normally come with integral relief valves. Would this solve your problem?

 

[itdepends]

Andy- thanks for the extra information. Based on what you've said above I'd say your in for another safety valve downstream of your HX.