Sealing a Problem Product

[Milkboy]

I have a pump which required
a sealing solution

940 mBar Vac Suction
2 Bar Discharge
1500 RPM

Pumped Product
140°C
30 cP
1.2 SG
Solidifies at less than 100°C

Im leaning towards a Metal Bellow
seal with SiC v SiC faces in a Cartridge
with a Steam Quenching Connection
with a retained stationary face
(stops the steam moving the faces)

Currently the pump has a steam pipe
pointing at it with a blanket
over the seal area keep the product warm
( >100°C)

Any Thoughts on this offer ?
What Steam Pressure / flow rate should be used ?
Is steam 'in at bottom / out at top' or the the other way
around ?
SHould there be a Steam Quench running for a period before
start up



Thanks



-
Milkboy

 

[longeron]

First- Steam Quench
The quench should be on at all times, not just when the pump is operating. And it should certainly be on before the pump is started. The purpose of the quench here is to keep the atmospheric side of the seal hot enough so that the product doesn't solidify and cause the seal to hang-up. Steam quench doesn't have to be at very high pressures, just enough to keep the temperature up. In the applications I'm used to I specify no more than 7 psig for the quench. If you really want to get into the flow rate, you'll have to perform some heat transfer calculations.

Second- the seal

As I mentioned above, the steam pressure does not have to be very high. So need to retain the seal faces to protect them from reverse pressure is gone. You might want to query your vendor about a steam guide of sorts to direct the steam in such a way that the fluid is pushed away from the dynamic elements- not packed into them.

Thirdly- Sealing a Vacuum

Do you have an estimate of the stuffing box pressure? It's my understanding that sealing a vacuum is dificult and should be avoided. Perhaps your box pressure will be above atmospheric. The problem is that air will want to flow into the seal/pump lubricating, if you will, the seal faces. Air in the pump is an obvious problem- cavitation and vapor lock (Of course there are exceptions for gas seals- such a small volume). Air in the seal is a problem because air is such a poor lubricator (Again gas seals are the exception).

I wonder if anyone has experience with a dual seal in this kind of service. The advantage of a dual pressurized seal in vacuum service is that the barrier fluid will flow into the lower pressure seal chamber and lubricate the seal faces. The barrier fluid will also lubricate the outboard seal faces keeping air from leaking into the pump. In this specific application a heat transfer fluid and heat exchanger could be used to heat the seal so that the fluid in the seal chamber would resist solidifying. It is possible that a heat exchanger would be unnecessary as the heat generated at the seal faces might be adequate to keep the fluid at the proper temperature.