Heat Exchanger Relief Valve sizing
Heat Exchanger Relief Valve sizing
(OP)
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.
RE: Heat Exchanger Relief Valve sizing
That depends on a lot: height of system, location of the relief valve in relation to high point, max pressure of all equipment. the pressure in the basement needs to be high enough to be above atmosphere at the highest point. but not so high, that at some point it is above the allowed pressure. Every 2.31 feet of height you lose or gin 1 psi if this is pure water. So if the basement has 40 psi, at 23 foot height, you only have 30 psi. but 23ft below the basement, you have 50 psi.
so thsi is a bit more complex than someone on the internet just telling you a number.
RE: Heat Exchanger Relief Valve sizing
RE: Heat Exchanger Relief Valve sizing
Cold fill pressure of 40 psi sounds high assuming this is a low rise building. But again, depends on at what height.
Yes, a relief valve located at the XHX and a setting lower than the HX pressure will protect the HX. But that may not be correct for the rest of the system. You cold over-pressurize a different component, or the relief valves open all the time....
RE: Heat Exchanger Relief Valve sizing
RE: Heat Exchanger Relief Valve sizing
I can't speak for the rest of the HVAC designers, but I don't see relief valves at the wrong location and the inspector would see that. They look for that. I see many HVAC design and installation problems, but relief valve locations isn't one of them.
btu-rating is based on the capacity of the HX. For an HX this is a value that depends on the fluid temperatures and flowrates, so that isn't a number just written on it (like for a boiler that has a gas line limit rating)
RE: Heat Exchanger Relief Valve sizing
(a) Steam to Hot Water Supply. When a hot water supply is heated indirectly by steam in a coil or pipe within the service limitations set forth in HG-101, the pressure of the steam used shall not exceed the safe working pressure of the hot water tank, and a safety relief valve at least NPS 1 (DN 25), set to relieve at or below the maximum allowable working pressure of the tank, shall be applied on the tank."
RE: Heat Exchanger Relief Valve sizing
RE: Heat Exchanger Relief Valve sizing
RE: Heat Exchanger Relief Valve sizing
RE: Heat Exchanger Relief Valve sizing
RE: Heat Exchanger Relief Valve sizing
RE: Heat Exchanger Relief Valve sizing
RE: Heat Exchanger Relief Valve sizing
This only needs a very small thermal relief valve set at 40psig.
The steam is unable to provide enough temperature to boil the water at 40psig.
If you have no trapped volume then you don't need a relief valve.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Heat Exchanger Relief Valve sizing
EnergyProfessional, your response is too surgical. The OP is trying to reverse-engineer this system and make a relief valve selection. Although you are correct from the perspective of a new design that volume has nothing to do with relief valve setting, how can this be true with an existing system? You have a given volume and a given expansion tank size. There will be a give thermal expansion which will result in a pressure increase. The relief valve setting will have to be higher than this or it will pop during normal operation.
LittleInch, how is it that a relief valve is not needed regardless of isolation valves? There is a heat-producing piece of equipment that is heating up a closed loop of water. What if the expansion tank floods and the system fills with cold water and then starts heating up? There will be no where for the expansion to go except a rupture. Similar situation if the steam valve gets stuck open.
Correct me if I'm wrong, but that's how I see it.
RE: Heat Exchanger Relief Valve sizing
Once expansion tank functions properly and is sized correctly (which it should), volume is not relevant. Pressure is pressure, regardless of if you have a gallon, or the entire Pacific Ocean in the system. A well designed system will never experience the relief valve opening except for testing. It is a safety valve that will open to prevent catastrophic failure in case your expansion tank fails (broken bladder etc.), your heat source runs amok etc. it is not supposed to open up under normal operation. Not ever. it is like an airbag in your car, if it deploys, something went wrong....
I recommend starting with this book
RE: Heat Exchanger Relief Valve sizing
Out problem is that we can't see the whole system and the OP hasn't posted any sort of diagram as to where the trapped volume could be.
Expansion tanks normally come with relief valves or there may be one somewhere else. We don't know.
Even if there is a locked in volume, the system a described only needs a small thermal relief valve as the steam temperature cannot over pressure the water side due to steam production.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Heat Exchanger Relief Valve sizing
Otherwise, you might as well just select the relief valve pressure based on the pressure rating of the system components.
RE: Heat Exchanger Relief Valve sizing
Relief valves have a Btu/h rating that corresponds to the volumetric flowrate they could discharge. This depends on the heat source maximum capacity to produce heat. And a pressure rating... that, as the name implies, is based on pressure rating. To size the valve, you need to determine both values. Not more, not less.
The relief valve also should be piped to a drain so you don't spill boiling water over all the people around it and not to make a mess when you test it. They should be tested monthly, and that also gets the dirt out.
I didn't think a relief valve can cause that much uproar. They have been successfully used since the time insurances got fed up with frequent boiler explosions. It is also one item inspectors actually look for.
RE: Heat Exchanger Relief Valve sizing
RE: Heat Exchanger Relief Valve sizing
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.
RE: Heat Exchanger Relief Valve sizing
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....
RE: Heat Exchanger Relief Valve sizing
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.
RE: Heat Exchanger Relief Valve sizing
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.
RE: Heat Exchanger Relief Valve sizing
RE: Heat Exchanger Relief Valve sizing
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.