Vacuum pump piping

bigpete638

So an accumulator tank is necessary?

Pumpguy

@bigpete638, Actually no. An auxiliary accumulator tank is a workaround to cope with a situation where the return line is below the inlet of the vacuum condensate pump.

A better arrangement is to locate the vacuum condensate pump in a pit so the condensate will gravity flow into it in the usual manner.

The attached file says "When it is structurally impractical......". I guess this means somebody doesn't want to go to the trouble or expense to construct the pit.

jumper

I don't understand why a water jet vacuum generator would be bothered by steam?

Harry_6

You are absolutely correct! An accumulator tank is obviously the way to go if you are doing an installation from scratch, or rebuilding. In this gent's case I was just suggesting that if there was a pipe elevation problem (like what is that new line running along the floor and back up?) there might have once have been a lift fitting. In the old days they had no problem with the use of them (probably because a second tank and receiver was expensive!). Of course if the job was designed properly originally you wouldn't need one at all.

By the way, although Hoffmans are OK (this being the 21st century and all), I personally think the Nash-Jennings was one of the finest pieces of engineering, ever. The method of adjusting air rotor clearance by unlocking and rotating the thrust bearing was a work of genius, and obviously designed by someone who had actually had to work on machinery!

But back to the original post: Can it be summed up - whether you are able to draw vacuum or not (although it's kinda expensive to use a vacuum/condensate pump as just a condensate pump) - all of the returns slope to the strainer (never lower), and the discharge goes . . .to the discharge. If you need more info, the VLR info materials are all available as PDFs online for free.

And the traps probably should be rebuilt asap, just to prevent further deterioration of stuff and all the potential problems created by live steam back feeding into things. How's that?

bigpete638

We’re actually replacing all the f and t traps with inverted buckets. We did a couple dozen so far but still a way to go. And that’s just one building. About 40 more to go in this building. Apparently the supervisor found a bargain on these stainless traps.

Pumpguy

@jumper, Steam causes cavitation damage to the centrifugal pump's impeller, the impeller that pushes water through the venturi nozzles to produce the vacuum. I have seen them torn up so bad, they look like a wagon wheel with no rim, just a hub and spokes.

@Harry_6, The design of Nash Jennings Vacuum Heating Pump you are referring to was in production from around 1917 through the mid 1950s. It had the vacuum rotor and condensate impeller mounted in tandem and spinning together on a common shaft.

Attached is a file showing this design.

Originally, there were no switches fitted. You just turned the pump on in the Fall, and off when the boiler fire died in the Spring. The large re-greasable bearings allowed for continuous operation, so a lift fitting, if needed to drain low returns was okay back then. Later on, float and vacuum switches were added to give start-stop operation.

When the pump shut off, there was no difference in pressure between the low return and the vacuum pump's receiving tank, so the low return just filled with condensate until the pump started again.

To resolve this problem, an auxiliary accumulator tank with float switch was substituted for the lift fitting. The float switch turned on the vacuum pump whenever this tank filled.

Around 1950, mechanical seals began to be used in the pump industry, as were C faced motors for direct close coupled assemblies. This allowed for more compact designs that were cheaper to manufacture and sell. These design changes also caused a change from a single motor dual purpose pump to single purpose pumps, each powered by their own motors.

These days, most vacuum heating pumps of any size are fitted with single purpose pump and motor assemblies; 1 or 2 pumps for condensate pumping only, and 1 or 2 pumps to produce the vacuum. For producing the vacuum, these are usually water jet venturi types, although we use a more efficient liquid ring type vacuum pump.

For smaller vacuum heating pumps, dual purpose single motor pumps are still used. A selector valve changes flow from the outlet of the centrifugal pump to send water through the venturi nozzles, or direct to the boiler or boiler feed tank.

jumper

What am I missing? Doesn't the centrifugal pump draw water from a receiver? So how does uncondensed steam get to the
impeller?

>>Steam causes cavitation damage to the centrifugal pump's impeller, the impeller that pushes water through the venturi nozzles to produce the vacuum. I have seen them torn up so bad, they look like a wagon wheel with no rim, just a hub and spokes.<<

Pumpguy

@jumper, The condensate is very hot due to steam carryover, maybe 210*F., maybe hotter. The additional energy added by the impeller causes the condensate to flash into steam inside the pump's volute. This causes cavitation damage that tears up pump parts

See TLAOSH, Pages 186 & 187.

jumper

Pumpguy said:

@jumper, The condensate is very hot due to steam carryover, maybe 210*F., maybe hotter. The additional energy added by the impeller causes the condensate to flash into steam inside the pump's volute. This causes cavitation damage that tears up pump parts

See TLAOSH, Pages 186 & 187.

Roth Pump used to advertise condensate stations that could handle boiling water. Don't your liquid ring pumps like Nash need cooling? Those Skidmore or Domestic venturi units should also offer a method to cool the water. Isn't the water pretty hot even when all traps operate perfectly? Water jet powered venturi work much better with cold water.

Pumpguy

@jumper, IIRC, Roth pumps are positive displacement turbine types, not common centrifugal. These may be able to handle hotter condensate than centrifugal types, IDK.

There are several centrifugal pump manufacturers that make 2' NPSH pumps specifically designed to handle hot condensate. These have an axial flow prop ahead of a conventional centrifugal impeller. Shipco is one brand that comes to mind.

A liquid ring type vacuum pump is a positive displacement gas pump. They need water, but for sealing, not for cooling, The pump uses the water like a piston to pump air; water being non-compressable. As the effective temperature of the pump goes up, it's air volume pumping capacity goes down. This is because much of the displaced volume is taken up by vapor.

For a better explanation, go to our website www.nashjenningspumps.com and click on HOW IT WORKS for a graphic illustration of the liquid ring pumping principle.

That said, ALL vacuum pumps need cold, or at least relatively cool water to operate efficiently. The attached file goes into detail on how condensate temperature affects vacuum pump performance.