Bypass relief in a heat exchanger
Bypass relief in a heat exchanger
(OP)
Hello,
We are designing over-pressure protection in an oil cooler, shell and tube heat exchanger to protect against high pressure drop during cold start up. This is to protect against damage to the baffles in the heat exchanger by supplying a relief valve around each baffle. We estimate that our baffles can withstand a 45 psi differential without failing, so we were planning a pipe loop around the baffle to the next with a 45 psi opening pressure. My question is - how do I add up the pressure drops in series, and account for the normal flow within the heat exchanger? For instance, if we test this unit, we will run the flow up (150 ssu lube oil) until we get 45 psi diff across the first baffle. At this point, the first bypass valve will pop, bypassing the flow to the second crosspass of the unit. We have 5 crosspasses in the heat exchanger (3 central cross passes and one under each nozzle). Will they all pop at once, or will it take an incremental increase in flow to get each additional one to pop? What will the flow vs pressure drop curve look like? If it was just one relief valve, it would be a linear rise and then flatten out after the valve opened, but what about three in series?
Thanks!
We are designing over-pressure protection in an oil cooler, shell and tube heat exchanger to protect against high pressure drop during cold start up. This is to protect against damage to the baffles in the heat exchanger by supplying a relief valve around each baffle. We estimate that our baffles can withstand a 45 psi differential without failing, so we were planning a pipe loop around the baffle to the next with a 45 psi opening pressure. My question is - how do I add up the pressure drops in series, and account for the normal flow within the heat exchanger? For instance, if we test this unit, we will run the flow up (150 ssu lube oil) until we get 45 psi diff across the first baffle. At this point, the first bypass valve will pop, bypassing the flow to the second crosspass of the unit. We have 5 crosspasses in the heat exchanger (3 central cross passes and one under each nozzle). Will they all pop at once, or will it take an incremental increase in flow to get each additional one to pop? What will the flow vs pressure drop curve look like? If it was just one relief valve, it would be a linear rise and then flatten out after the valve opened, but what about three in series?
Thanks!





RE: Bypass relief in a heat exchanger
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Bypass relief in a heat exchanger
RE: Bypass relief in a heat exchanger
Assuming a liquid and near constant viscosity, there is a flow rate that represents 45-psi delta p across a single baffle. Because the baffle spacings and baffle area window are approx equal, the same flow would generate 45-psi delta P across all baffels.
A single control valve across the entire shell set to open for > 45-psi would be my recommendation.
RE: Bypass relief in a heat exchanger
This is a relatively small heat exchanger and the bypass relief is actually in the baffles, so the external is only a representation. You've answered my question. Pressure drop is a function of flow rate, so the flow that pops the first one will pop them all.
RE: Bypass relief in a heat exchanger
I don't understand why the upper baffles don't have similar relief valves, but maybe that isn't important.
Yes I agree with Yitbos, the same flow would tend to cascade all relief valves. After cascade the slope of your graph above would correspond to the K factor of a valve, provided that all other flow through the baffels was blocked. Otherwise there would be a slope representing the combination in parallel of any dP from flow still going through the baffles with the dP predicted by the K factor of a valve.
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Bypass relief in a heat exchanger
rmw
RE: Bypass relief in a heat exchanger
Ted
RE: Bypass relief in a heat exchanger
and more reliable.
Keith Cress
kcress - http://www.flaminsystems.com