NPSHR for pregressive cavity pumps

[sb00]

Hi folks,
does API 676 apply to progressive cavity pumps?
I want to know it with respect to the cavitation criteria of 3% drop in flow or head.
Do progressive cavity pumps also follow same cavitation criteria (for NPSHR calculation) of 3% drop in flow/head?
If not, what is the cavitation criteria for these pumps?

 

[zdas04]

PCP's rarely fail due to cavitation. Mostly they turn far too slowly for the cavitation dynamics to come into being.

That being said, the pumps have a very high head. In a 3,000 ft well with 20 psig flowing bottom hole pressure and a discharge stream water-full, the head is 1,273 psi. If you ever fill the barrel with gas then the pump will try to turn into a compressor with 38 compression ratios. Heat of compression (with k=1.28) is over 1,000F and the stator will coke within seconds.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

 

[REFINERYPROJECTS]

No, It doesn't.
API 676 FOR POSITIVE DISPLACEMENT ROTARY PUMPS

Cavities are another name for a hole in a liquid, called bubble, so cavitation is all about bubbles forming and collapsing. Bubbles take up space so the capacity of our pump drops. Vapor pressure is about liquids boiling.

If you know the temperature of your product you need to know its vapor pressure to prevent boiling and the formation of bubbles.
• Specific gravity is about the weight of the fluid. Using 4°C (39° F) as our temperature standard we assign fresh water a value of one. If the fluid floats on this fresh water it has a specific gravity is less than one. If the fluid sinks in this water the specific gravity of the fluid is greater than one.
• Look at any pump curve and make sure you can locate the values for head, capacity, best efficiency point (B.E.P.), efficiency, net positive suction head (NPSH).
• Head is the term we use instead of pressure. The pump will pump any liquid to a given height or head depending upon the diameter and speed of the impeller. The amount of pressure you get depends upon the weight (specific gravity) of the liquid.

NPSHR is defined as the NPSH at which the pump total head has decreased by three percent (3%) due to low suction head and resultant cavitation within the pump. This number is shown on your pump curve, but it is going to be too low if you are pumping hydrocarbon liquids or hot water.
Cavitation begins as small harmless bubbles before you get any indication of loss of head or capacity. This is called the point of incipient cavitation. Testing has shown that it takes from two to twenty times the NPSHR (net positive suction head required) to fully suppress incipient cavitation, depending on the impeller shape (specific speed number) and operating conditions.

 

[Scipio]

Nope, API 676 does not necessarily apply to progressing cavity pumps. They're basically built to manufacturer's standards, though some manufacturer's circle around API 676 just as a starting point for their own design specifications.

As far as NPSHR testing, since PCP's are really positive displacement pumps, Net Positive Inlet Pressure (NPIP) requirements is a more appropriate term for the suction requirements. The actual testing procedure varies from region to region and manufacturer to manufacturer, it's not as standardized as centrifugal pumps. Test curves are generated based on anywhere from 2% (German VDMto 5% change in head, discharge pressure, or flowrate with PCP's (whereas centrifugal pumps are a 3% drop in TDH). Just to make things more interesting it means a lot less, since as a rule PCP's aren't as vulnerable to cavitation damage as many are constructed for multi-phase flow.

Long story short, ask the vendor how they test, chances are fair every vendor you ask has different criteria for generating their NPSHR/NPIPR curves when it comes to progressing cavity pumps. You might also want to check with Seepex, Roper, Mono and Moyno, I know Seepex at least has published a couple papers on the matter.

 

[PUMPDESIGNER]

REFINERYPROJECTS,

Your understanding of cavitation is close but missing something.

Cavitation involves a phase change in the liquid, from liquid to gas and back again.

All other gas bubbles occurring in a liquid form by other means and are vastly different. Entrained gas bubbles can come out of solution or back into solution, and this is by means of diffusion which is a slow and low energy process.

Cavitation forms voids that appear to us as bubbles, but are as I stated very much different than other bubbles. Cavity formation and collapse is very fast, much less than a second. Cavity collapse is extremely violent and can destroy pumps even in small amounts. All reliable authorities report that cavity collapse causes temperatures reaching as high and even exceeding 100,000 degrees F, and pressures of 10,000 psi, some report 20,000 psi and higher.

Yup, all that on a microscopic scale and in normal cavitation occurring every day in millions of pumps world wide.

But the phenomena I just reported explains why cavitation sounds very different than entrained gas, and why cavitation eats metal and plastic, while entrained bubbles doe not.

Some materials withstand cavitation better, Type 316 stainless is the best of all easily affordable metals, cast iron is the weakest of all metals.

PUMPDESIGNER

 

[smckennz]

It is difficult to understand how incipient inlet cavitation would produce a small (e.g. 3%) total head loss in a PD pump as they are positive displacement, and the discharge head is therefore essentially independent of the pump. Perhaps there may be a slight reduction on flow, if the cavitation area extends into the cutoff zone (where liquid can no longer flow back to the inlet).
As PD pumps are often used to pump viscous liquids, it is not unusual to hear them "growling". I assumed this noise was some form of cavitation but never gave it much thought.
Are there any published standards on the suction requirements for progressive cavity pumps?
I have designed the inlets for a number of reciprocating pumps, but always from first principles and with probably excessive conservatism .
A standard sure would help.

Cheers

Steve

 

[Scipio]

Smckennz,
Never been 100% on that one myself, actually. The Hydraulic Institute, for one, actually has some guidelines for NPSHR testing on PD pumps, though they formally refer to it as NPIPR on PD pumps. The combined ANSI/HI standard for rotary pump tests, ANSI/HI 3.6-1994, states;

Normal NPIPR tests are conducted in a test environment that minimizes entrained gas. NPIPR is established at the first indication of any of the following:
1) A 5% reduction in capacity at constant differential pressure and speed;
2) A 5% reduction in power consumption at constant differential pressure and speed;
3) The inability to maintain a stable differential pressure and speed;
4) The onset of loud or erratic noise when this criteria is previously agreed upon by all parties.

The equivalent ANSI/HI standard for recip testing just sets it at a 3% loss in capacity at a specific pressure and speed.

I'm assuming the performance losses observed in PD (rotary or reciprocating) pumps will be primarily due to the compressibility of the vapour bubbles reducing the volumetric efficiency of the, reducing the observed power or capacity of the pump, rather than the normally observed impact on TDH that a centrifugal pump displays during cavitation. A NPIPR problem I've faced a few times with reciprocating pumps, for instance, isn't as much physical cavitation damage as it is capacity reduction and vapour-locking due to the presence of vapour & entrained gas dropping out in the liquid end.