Ethylene Transfer Pumps

[carlos1602]

We have two pumps in ethylene service at a Styrene Monomer plant. The pumps are vertical submerged type, 17 stages with an NPSHa of 0.5 m. to run the pumps we have to have the storage tank level above 9 m static head or the pumps will cavitate and trip on low flow. We need to get the tank level down to about 1.5 m as having too much stored ethylene means that we cannot sell what we do not need to store. As the boiling point of ethylene is -103 oC and have been advised that the tank storage temperature should be -10 oC colder. We doubt the integrity of the pumps and the pumps worked well up until a while ago then have gradually decreased in efficiency. Also, we have had the issue that the suction temperature at the pump increased to -94 oC so with the corresponding vapour pressures being converted to metres of head, we saw and increase of some 7+ metres. Any suggestion what we can do to improve these pumps?

 

[1gibson]

Ask vendor about the first stage impeller design.

If you originally specified an Nss limit, that would force them to use a "standard" series impeller as the first stage.

If that was the case, you could replace with a higher Nss (which directly means it will have lower NPSHr) dedicated first stage impeller and that would lower your NPSHr by a few feet.

A bit of a longshot, but pretty likely if an engineering firm got their hands on the project.

 

[dcasto]

It sounds like there is excessive wear in the internal seals between the stages and ethylene is slipping back to the suction. At -94C and asumming you started with saturated liquid and 7 meters of head with 9C temp increase, you would have 100% vapour in the pump suction. I'll guess that the pump has about a 5C increase in temperature across it.