Heat Exchanger Relief Valve sizing

[Cool_Controls]

I have come across a shell and tube heat exchanger in the basement of my office building that has no pressure relief valve on it and I would like to know how to select the right size relief valve for it. The heat exchanger provides hot water for the buildings HVAC system, 15psi steam on the shell side, 40psig water in the tubes, centrifugal pumps. Its always been my understanding that the relief valve in this context protects against thermal expansion either from a blocked in condition or failed expansion tank. A tube failure on these units just results in heating water being lost into the condensate line and making the boiler plant operators annoyed at the hard water coming back to the plant.

 

[LittleInch]

In this case the heat source is limited to the temperature of steam at 15psig as this is regulated and has a relief valve.

This temperature is 120C.

If the water side is capable of at least 40 psig, it would take a temperature of 140C to boil the water. So you don't need a relief valve capable of discharging steam. You only need a small 1" relief valve to take care of the water expansion.

Be aware though that any water coming out could flash into water and steam so make sure the valve exit is piped somewhere safe.

If the operating pressure is 40psig, then the design pressure / MAWP should be higher and this is what you set your relief valve to. that is

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EnergyProfessional]

Boiler manufacturers request that information from the engineer (submittal review) or go by what is on the plans. The boiler manufacturer will have exactly zero information about the system, that information comes from the engineer who determines the correct pressure.

You need to account for the elevation of all equipment. 40 psi will be less if higher, and more if lower. You will not get around looking at the entire system from lowest to highest point. It isn't as simple as converting a temperature to a pressure. Pressure changes with elevation....

 

[LittleInch]

There is more than one post here which starts "If this was a boiler...." or "A boiler...."

This is a heat exchanger. It is not a boiler.

The two are very different in terms of what sort of pressure relief system you need.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TBP]

I didn't word my post very well - hot water boiler makers will provide the relief valves for the rating of the boiler specified. I have seen hot water systems that ran at 350 PSIG. But the boilers were the boilers, and the system had any overpressure protection required that was separate.

The steam safety protects the hot side of this HX from seeing what would be pressures over 15#. Without a steam safety valve - if the steam control valve malfunctions - it would be possible for the hot side of the HX to see whatever the line pressure is that supplies this building. That could be 150# or more. On the systems I have seen, this would be 366*F, instead of 250*F max on a low-pressure steam system. You can suddenly have a lot more heat input into your hot water heating system than anticipated or desired, along with the HX that is not likely rated for 150# steam side.

 

[TBP]

Regardless of whether it is a hot water boiler, or a steam to water heat exchanger - both require the same kind of over-pressure protection on the water side.

 

[LittleInch]

Sorry but they don't. A fired boiler is quite different to a steam water heat exchanger.

If the steam is well controlled and has a relief system like this one has, there is a limit to the temperature of the water and if the design pressure / MAWP of the water side is high enough, there isn't a releiving event other than thermal expansion which only needs a small valve.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.