Loss of Prime at H2O Boiling Point

[jbkiser]

We have a quench tower that is circulating diesel fuel as the cooling fluid. The gas being cooled in the tower has some water vapor in it and
we typically run the diesel oil high enough that the gas never cools below 230, so it stays in the vapor phase and we remove the water further downstream. When we startup however, we start at ambient conditions where the gas is initially cooled below 212 and the water condenses into the oil. This shouldn't be a problem because once we get to our typical operating temperature it would simply boil off.

But we do have a problem right at the transition point. We have run a number of tests and see our diesel pumps consistently begin erratic pumping at about 220F and at about 230F stop pumping altogether. We think what's happening is the pressure drop at the pump suction is flashing liquid to vapor and our centrifugal pumps are simply spinning in water vapor. This seems to be worse as we approach a filter changes suggesting the back pressure across the filters is making matters worse. Even after a filter changes, we have trouble recovering consistent pump volumes unless we let the oil cool back down.

Has anyone run into this before and have a suggested solution to prevent this?

Thanks

 

[Compositepro]

Filters should be on the discharge of pumps not on the suction for this and other reasons. If the pumps need protection, a coarse strainer on the suction should be sufficient. A water separator and drain prior to the pump might help.

 

[lilliput1]

What is the flash point of the diesel fuel being used? I don't think it can be above 230F. If it is 205F then you can not prevent cavitation and you will be producing an explosive situation.

 

[jbkiser]

Compositepro - the filters are on the discharge side of the pumps. Could possibly use a separator but I'd prefer a remediation step to apply at the transition point because once we get to a high enough temp, its no longer a problem.

lilliput1 - The flash point of diesel is about 205F but there is no air in the system and no ignition source so there's not an explosive situation. The boiling point of diesel components should be well 350F so the diesel shouldn't be causing the vapor formation. We believe it's water.

 

[lilliput1]

You need to make sure the available NPSH exceeds the pump minimum requirement. You should use a high temperature heat transfer fluid like Therminol with 338F flash point instead of diesel.

 

[georgeverghese]

Change the startup operating procedure to flow gas into this quench tower at a much lower initial rate and allow the hot gas rate to ramp up (via process controls) only when quench tower exit diesel is at higher than 230degF?