Submergence requirement vs NPSH margin for Vertical Pumps (VS1) 1

[PrSK]

I have to finalize pump model for Vessel mounted Vertical pumps - VS1 type. The vessel has boot with dimensions such that minimum submergence requirement for pump shall be maintained within boot.

The fluid is HC with pumping temp. of ambient to 185°C.
The vapour pressure is 1.2 barg and suction pressure for pump is 1.3 barg giving NPSHa of 1m.

Now I have two proposals,
In first case, the minimum submergence of 650mm (from 1st impeller) and NPSHr value is not mentioned stating that pump will run till 650mm and shall stop at 659mm.

In second case, the value of NPSHr for pump is 1.7m.
Minimum submergence is calculated considering 1m margin over NPSHr as 2.7m.

And the diameter of vessel is 1.8m.

Which of the above is correct proposal?

 

[vpl]

First is this homework?

Second, why don't you ask the manufacturer of the first proposed pump what is the NPSH_r rather than trying to guess. This would be a reasonable thing for a pump supplier to supply. As a customer, you can request this.

Third, besides NPSH_rand NPSH_a, you really need to figure out if you have proper submergence, which usually gives you a height above the minimum requiredNPSH_a.

Fourth, look at your available NPSH and the required NPSH for the second proposal. Think about it a bit. Is this pump at all reasonable for your application?

Patricia Lougheed

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[JJPellin]

A vertical pump can have a minimum submergence and a net positive suction head required. The two are independent and depending on the service, one or the other could be the limiting factor for the design. The NPSH(r) is the minimum suction head to avoid cavitation (beyond 3% head loss) in the suction eye. The minimum submergence is the minimum requirement to avoid a vortex that could introduce vapor (usually air) from above the liquid level. In our cooling tower applications, we have pumps that have adequate NPSH, but inadequate submergence. We have other pumps that have adequate submergence but inadequate NPSH. As already noted, you should ask the pump manufacturer to specify both. They may only be giving you the one that is the limiting factor based on the information you gave them.

Also keep in mind that the NSPH(r) is given in absolute units. Submergence is given in relative units. If your product is at equilibrium at the liquid surface, it works out the same. But this is not always the case.


Johnny Pellin

 

[PrSK]

Thank you vpl. Thank you JJPellin for excellent summary.

I am trying to select pump for this duty.

HC in the the vessel in which this pump needs to be fitted, is boiling. Like JJPellin has pointed out, yes the product (HC) will be in equilibrium considering boiling fluid and vapour pressure of 1.2 barg.

hence, NPSHa and suction pressure is only static head of fluid above the first impeller.

Two questions,
One, For vertical pump, considering the impeller always under fluid, how significant is the margin requirement between NPSHa and NPSHr?

Two,once minimum submergence is calculated, is it necessary to reconsider the margin?

In any case, how the selected pump will behave when starts working?

 

[JJPellin]

The fact that the impeller is submerged has no affect on the required NPSH margin. For HC service, we would require a margin of 3 feet (1 m) as a minimum. Even submerged by the appropriate amount, the impeller can still cavitate. Even if you satisfy the minimum submergence to avoid vortexing, you still need to verify that you have the minimum 1 m NPSH margin for NPSH(a) above NPSH(r). Both of these criteria have to be evaluated and both have to be satisfied.

I should also warn you that this style of pump is very sensitive to the configuration of the sump or can it is in. If there is any pre-rotation of the incoming fluid, the pump may cavitate even if it satisfies the requirement for NPSH margin and submergence. In open sumps, there is often a stationary ridge or vane in the bottom of the sump to break-up any pre-rotation. In the vessel you are describing, this is probably not required.


Johnny Pellin