plotting NPSHr vs NPSHa info

[ebermudez]

My boss told me to plot a NPSHa curve on the fabricant NPSHr chart, what information could give me both curves together?

We're analyzing this pump, beacause it's causing much troubles, apparently due gas appearance. We are pressuming vortexing, though my boss wanted to check the NPSH.

I will appreciate your support!

New grad - EB

 

[checman]

ebermudez

NPSHR and NPSHA change with flow rates. A rough curve can be made from maximum flow minimum flow and average flow along with the corresponding NPSHR and NPSHA.

 

[ebermudez]

I've already plotted both curves.
Considering the worse condition -> Ho = Hvp ....(NPSHa= Ho - Hvp +/- Z - hfs), the vapor pressure = pressure inside the vessel.
At that critical condition the NPSHa would never be greater than NPSHr according to the provided NPSHr curve, can I assume that the gas appereance problem is because of vortexing?

 

[rmw]

What is your definition of "vortexing?"

rmw

 

[Artisi]

"Vortexing" has nothing to do with NPSHa/r.

I feel that you have a problem with your terminology, if you have entrained air / gas in the pumped liquor this is not "vortexing" but simply air entrained product. Entrained air/gas can put the pump off performance depending on the amount of air/ gas in the product being pumped.

Is this the same pump and is it connected to the question you asked in a previous posting regarding the impeller diameter, if it is -- I suggestion you combine the 2 or more problems you are faced with into one question so that we may help you.

Naresuan University
Phitsanulok
Thailand

 

[ebermudez]

Artisi, yes it is the same pump. I made the previous question about the impeller diameter, in order to estimate operation flow value, because i had the fabricant charts, but the nameplate on the pump wasnt legible to see the impeller diameter and choose from one of the diameters/specific velocity curves.

rms, I mean vortexing, when the velocity at the suction pipe on the drainage becomes high and the fluid swirls (excuse me if my english isn't the best, and correct if im wrong) so create a vortex and gas can entrain to the pump. This usually occurs when the submergence level over the inlet to the suction pipe is not adequate.

NOTE: This pump is used to drain the condensate of naphtha in a scrubber. The scrubber doesnt have a vortex breaker, and i personally think that the level switch is not working well, so the submergence level could be significantly low providing vortex formation.

 

[ebermudez]

Artisi, howelse could the gas entrain?....as well as vortexing.

Thanx for your interest

EB

 

[ebermudez]

Since the curve of NPSHa that i determined is below the NPSHr, how could that pump be working over that condition of cavitation???

I was reading about NPSH reduction for hydrocarbon liquids and hot water applications, and because the sheets provided by the manufacturer are made for cold water operation, some considerations must be taken given that the fluid behavior differs over cold water.

So this mean that because this pump is handling a hydrocarbon fluid (naphtha), the pumps operate under this condition for its NPSH reduction?....if not how could this happen?

 

[sprintcar]

Air can become entrained in the sump from the fluid entry.

Unless the supply pipe is fully submerged, splashing results and this will carry air into the fluid in the sump. Turbulence resulting from filling the sump can also mix air into the fluid. Depending on the conditions, some of this may easily find its way into the suction inlet for your pump.

The vortex condition can usually be visually observed, and adding a plate over the suction inlet area can reduce it, as can raising the level of your sump.

"If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut."
-- by Albert Einstein

 

[Artisi]

It is important to differentiate between air entrainment and a vortex forming at the pump inlet. These are separate conditions which are unrelated to each other.

As described by sprintcar in the previuos posting, air entrainment is usually introduced either by the process or by the supply of product into the inlet tank / basin / reservour / etc.

A vortex forming at the inlet to a pump can be a complex phenomenon. The most understood cause is not having sufficient submergence of the inlet or inlet pipe leading to a vortex forming at the surface of the supply tank, although a vortex can sometimes form when there is more than adequate sumbergence - this is usually easily cured. However, inlet vortexing is possible from other than forming a vortex to the free surface of the pumped product, a vortex can be from the bottom / side walls of the tank which in many cases is not observed and can be a complex problem to overcome, it is too involved for discussion here but if interested you should be able to search out information on the web.



Naresuan University
Phitsanulok
Thailand

 

[rmw]

Check with KSB and see if they have any technical references, installation manuals, or papers regarding sump dimensions, specifically distance from side walls and rear wall, minimum submergence required for your model pump, and any recommended or minimum height of the pump suction bell from the bottom of the sump. Some vertical pumps are very sensitive to any or all of these parameters.

rmw