Calculating Force Absorbed by Seal Faces (ultimately the seal gland)

[GaTechTheron]

There are actually a few questions in this one:

I have a high suction pressure application. Infact I will need to use a larger motor because of the upthrust limitations on the bearings (Vertical Turbine with Can). My questions are as follows:

1. Is there a way to calculate the force absorbed by the seal faces (Basically, this should be the force absorbed by the seal gland).

2. Can we subtract this number from the total upthrust to avoid upgrading to next largest motor size? Is this good practice? Or should we always assume max upthrust (i.e. all thrust is transferred to motor bearings) to avoid any possible issues in the future?

3. Is this the force used to calculate the hp loss through the seal?

Thanks for all of your support!

 

[1gibson]

1) You are in trouble if you bottom out the seal to the point there is a reaction force on the seal gland. Most seals will specify an allowable axial movement (+/- 0.015", for example, depending on seal type) and exceeding this will damage the seal. You can't use the mechanical seal as a thrust bearing.

2)Suction pressure acts on the seal sleeve area, and the shaft cross section (basically anything that physically comes out the top of the seal.) This is area with suction pressure on the bottom, atmospheric on the top, so you get upthrust of that area times suction pressure (gauge.) This reduces thrust (causes more upthrust.) ALL thrust is transferred to the thrust bearing.

3) HP loss through the seal should be minimal, unless you have a 15hp or less motor it should be negligible.


You may be able to alter the balance configuration of the pump impeller(s) to increase downthrust, check with the manufacturer. I assume you've already verified the pump will operate in upthrust without damaging lineshaft bearings...

Need more information: pump head/flow, fluid SG, seal type, piping plan, type of thrust bearings?

 

[GaTechTheron]

Thank you!

 

[Enkidu]

Your seal vendor should be able to advise both the axial thrust on the seal sleeve, as well as the starting and running torque requirements of the seal.

Suggest you ask them.
Good luck

 

[Scipio]

If there's enough thrust being generated to require oversizing the motor, I'd suggest getting away from a rigidly coupled design period - have the pump manufacturer install a thrust pot on the pump itself, switch from rigid to flexible couplings, and get the thrust away from the motor entirely. Depending on what size you're talking about, you may even find a pump side thrust bearing is the cheaper option up front. It also means you get engineered bearings rather than just whatever's in a particular motor frame.

Bear in mind that down the road, every time that motor has to be replaced, they need to buy another oversized motor, which may mean it's a more cost effective option in the life of the facility, even if it is more costly up front.