Single Mechanical seal in place of double seal for a VS1 Type Pump 1

[Reticular]

Just a quick question. Can single Mechanical seal be acceptable from a vendor over a double mechanical seal in pumps for a vertically suspended pump type, VS1. What are the implications in terms of cost savings and functionality.

 

[1gibson]

If it's a VS1, which is a sump pump (no barrel, that would be a VS6) then it is llikely pumping from an open pit. So the fluid shouldn't be too nasty, and a single seal will probably be ok.

Single seal will be cheaper than double, you don't need a gas or barrier fluid system, but you also don't get the possibility of zero emissions.

 

[Reticular]

@1gibson, thanks for your insightful response. I just got a response from the vendor that the cost savings is appreciable and I think with regards to functionality, the single seal should be ok.

You are right, it pumps from an open pit. However, some pumps are also specified and are expected to pump from a closed drain underground pit. I hope the VS1 type pump should be suitable in this regards as well? Do you think otherwise?

 

[1gibson]

VS4 will be cheaper if all it needs to do is drain the pit periodically, and those don't need a mechanical seal at all.

If your fluid is clean enough, you could do forced product lube (tubing from discharge to each bearing). If it is a dirty fluid, clean water injection, or grease injection. Beware of maintenance requirements for grease, needs a squirt every few hours of operation, so suggest you look into an automated solution. Use your mechanical seal budget for that.

I'm aware there are other applications for VS1 pumps, I might just be one of the better mind readers on the board but remember, nobody is perfect. Try to include a little more information, fluid identity would be a good start. Erosive particles (sand) ? If so, even more reason to go VS4 over a VS1, VS1 column and bowl bearings will not like particles.

 

[Reticular]

Thank you 1gibson... Your responses have been quite helpful and professional.

The pumps are at the bid stages and PO is about to be issued, just some technical clarifications from the vendors.

As per the fluid, the pumps are expected to drain from a closed drain vessel to a drain header. The pumped liquid in this case is condensate (wet hydrocarbon). Another one is expected to drain from a heat medium vessel, the liquid been pumped in this case is heat medium fluid ( Therminol 55). The third set of pumps, however will drain condensate from an open drain pit which will likely be covered with plates.

As regards the frequency of drain, I expect this to be a continous process as there is a standby pump in each case.

Do you still think the choice of VS1 is not ok? I will appreciate a quick response.

 

[1gibson]

You're welcome. VS1 does seem appropriate for those services. If head/flow are similar, and pumps have same shaft size, it might be possible to select a single seal that will work for all applications. Buy complete spare seals, and repair kits.

But generally anything with a drain pit nearby, you are wasting money on dual seals.


I once had a project with VS1 pumps in a drain pit, water, hydrocarbon, "small amounts of ssand." Plan 13 (bleed off line to "suction region" which in this case was just back down into the sump) and a plan 31 (cyclone separator takes fluid from discharge, clean flow to seal, dirty flow to "suction region".) So when running, there are these two steady streams of fluid spraying off back into the sump. The customer specification also required a plan 65 drain pot, you know, in case the seal leaks more than a few drops they get an alarm! Couldn't explain to customer that the 65 was silly, and further, they would do perfectly fine using packing, saving $10K a pump in upfront costs. I don't even want to think about maintenance costs.

Those pumps SHOULD have been VS4, which would have saved them probably 20% or more over the VS1's they ended up with.

The price of common sense... I wonder if there is such a thing as a "specification review consultant." Because the customer can't ever trust the vendor to make suggestions right? The spec is the spec and any deviation is bad, especially when vendor suggests it.

Refrshing to see someone that cares more about getting the most appropriate equipment, than meeting a generic specification.

 

[PumpSmart]

There is some 3 main reasons why mechanical seal is suggest.
Safety, Toxic or Flashing Fluid.
First is recomendable that you know why vendor bid that to then undertood that there will be not problem using single seal.
VS1, VS4, VS6 pumps are not similar.
VS4 pump is a true sump pump design, single stage, lineshaft.
VS1 pump is a vertical suspended pump design, single or multiple stage, lineshaft.
VS6 pump is a vertical suspended pump design, single or multiple stage, lineshaft, double casing pump.
VS1, VS4, VS6 are covered in API610 standard.
VS1 is superior in engineering, design , pressure-capacity cpabilities than VS4 and similar to VS6.
In VS4, true sump pump design, no mechanical seal or packing is needed in general services. In special vapor proof services, mechanical seal is needed.

I hope this helps.