Hydraulic Properties for NPSH Calculations

[MEnut]

I am trying to calculate the NPSH (required) for a hydraulic pump to determine if it could be cavitating. However, I do not have any info on the properties of hydraulic fluid. For approximately 100 deg F, can anyone tell me what the vapor pressure is? Or am I doing this wrong????

Thanks guys!

 

[Scipio]

You can't calculate the NPSHr, it's a property of the pump itself and has to be determined by manufacturer testing. The NPSHa (available) in the system, on the other hand, has to be calculated. Is this what you mean? Basically if the pump requires more than is available in the system you get cavitation.

 

[MEnut]

Yes, my mistake. I am trying to calculate the available.

Thanks.

 

[25362]

Hydraulic fluids can be based on straight or synthetic lube oils, W/O and O/W emulsions, water glycol + polyesters mixtures, phosphate esters, silicate esters, glycol ether esters, etc.

Therefore, I think you should know the fluid's composition, viscosity, density, vapour pressure, flow rates, thermal properties and operating conditions, prior to start making a pump NPSH definition.

 

[TD2K]

If you know the name of the fluid, check with the manufacturer.

The MSDS should also give you at least the flash point (which luckily is usually at 100F). If you do a keyword search on this site for flash point, estimating the vapor pressure from that parameter has been discussed several times before.

 

[Scipio]

Designing a pump up front I tend to agree with 25362, but if you're already experiencing what you suspect may be cavitation you can probably be a little more basic with the calculations. Specific gravity, vapour pressure & viscosity should be enough for your purposes - have you checked with the manufacturer of the fluid for an MSDS? Might even be able to get enough info there to at least estimate NPSHa.

What makes you suspect the pump is cavitating, anyway?

 

[MEnut]

I was hoping to be a bit basic, to see if I am even in the ballpark of cavitation.

The pump I am looking at was making an awful sound, and I started thinking it may be cavitation, and a little practice on some fluid system equations couldn't hurt, right? But as I start to look into this, I began to realize that we don't always have the 'same' system. If the hydraulic filters are dirty, that changes the NPSHa. Also, we hook up an external filtering buggy to clean the system as it's running. With this, it changes the head losses on the inlet side, elevation of resorvoir, etc. So that can change the NPSHa. Turning into an interesting project. We have many other hydraulic pump applications in the facility that I should look at, too.

As for the for flash point, I'm not sure, but I thought that was it's combustion temperature? (I'm an ME, so cut me some slack on the jokes if I'm wrong!)

 

[d23]

MEnut:

Most hydraulic pump applications do not use 100% hydraulic oil. As 25362 stated you will have other additives that need to be taken into account when considering the fluid's composition or properties.

NPSHa and NPSHr both need to be considered. The NPSHr for hydraulic oil will be less than it is for water. The pump manufacture will provide you a curve based on water not oil. Once you get the fluid composition ask the manufacture what the NPSHr is for that fluid. You need to have the manufacture involved for liability purposes of new or rebuilt pumps.

One quick test for the noise and possible cavitation is increase the intake pressure when the pump is running just to see if it gets quiet. You should be able to do this by bypassing filters increasing reservoir level etc.

Good luck!

 

[TD2K]

The flash point IS its combustion temperature (hey, not bad for an ME ;-)).

But, for most heavy hydrocarbons, you need about 1% to 1.5 mol% vapor to reach the LEL which is what you are measuring with the flash point. Thus, the flash point gives you a vapor pressure (1 mol% in the vapor at sea level is about 0.15 psiA vapor pressure).

Duh, BTW, where's my coffee? Flash points are not done at 100F as I babbled above, don't know where my brain was this morning, obviously NOT working. If you can get some more information on your fluid, I can take a look at it for you if you still want an estimate of the vapor pressure.

And as you pointed out, dP across your filters will have a big effect on the NPSHA for the pump. Is the suspected 'cavitation' always there or does it come and go with specific operating conditions?

 

[25362]

The wide range of hydraulic applications necessitate a variety of pump designs including gear, vane, axial, and radial piston types with a variety of metallurgies. Since clearances in pumps and valves tend to be critical, it is important to provide adequate filtration (full flow or bypass, or both) to maintain the system in as clean a condition as possible, and thus minimize wear.

Ignitability of fire-resistant fluids is measured by methods such as ASTM D 5306, not by regular flash points as with petroleum-based lower-cost fluids.

Although NPSH considerations were developed and apply mainly to centrifugals, suction problems also apply to displacement pumps. May we know what type of pump you are referring to ?

On screw pumps the maximum suction lift available MSLA = 1 atm - NPSH, or in SI units = 10.36 m -NPSH. NPSH_r = atmospheric pressure minus MSLA.

Some manufacturers are reluctant to publish a single value of minimum inlet pressure required for satisfactory operation for fear that it may not cover all conditions encountered in practice, such as gas entrainment or temperature fluctuations resulting in viscosity changes and changes in line losses. Gases come out of solution when a pressure below that in the reservoir exists at the pump suction.
One of the apparent effects of handling liquids containing entrained air or dissolved gas is noisy pump operation. Such a condition is usually wrongly dismissed as cavitation. In fact the mixture of oil and air is compressible, the volume in each closure is reduced as it comes in contact with the discharge pressure. This produces pulsations and noise, the intensity and frequency of which depend upon the discharge pressure, the number of closures per revolution, and the speed of rotation.

Noisy operation and vibration are objectionable, and reflect, in fact, an inefficient pump performance that should be corrected. Proper pump/piping design and proper speed selection can go a long way towards overcoming the noise problem. d23 checking suggestions are to the point.