Pump Minimum Flow - mechanical protection 5

[tkdhwjd]

Folks,

I understand that a percentage of the process fluid will recirculate at the eye of impeller and the percentage increases as flow drops. Heat build up due to this recirculation can potentially cause flashing, and hence cavitation as fluid velocity turns into pressure.

I also heard about "mechanical protection" of a pump at low flow. Can anyone please explain why the pump is mechanically unstable when the flow drops below the minimum continuous flow rate?

Perhaps the magnitude of flow reversed (shift in momentum) is so high that the impeller gets exposed to significantly unbalanced force?

 

[Ashereng]

Centrifugal pumps have a NPSH requirement. Most pumps have a low suction pressure interlock to stop the pump when pressure is too low (or can't start the pump until pressure is sufficient).

Minimum flow bypass/recirculation is on the discharge side of the pump, and usually goes back to the tank from which the pump is drawing from.

 

[vanstoja]

Recirculation flow in a centrifugal pump inlet at off-design (particularly low) flowrates is akin to stall on an airplane wing when the flow incidence angle becomes too high. Most airfoil shapes stall with incidence angles of about 12 to 18 degrees. Heatup cavitation effects are only likely when pumped fluid temperature brings suction pressure close to fluid vapor pressure. At much lower fluid temperatures, the cavitation concern with recirculation flowfields is the very substantial vortex core pressure reduction in fluid vortices spawned by the near-chaotic recirculating flow. Possibly, flowrates below the range of "stalling" incidence angle will dissipate recirculation and become stable again. In the stalling flow range of recirculating flow, pump head becomes variable leading to transient variations in both radial and axial hydraulic thrust acting on the impeller as well as pump driver input power fluctuations. High pump stuctural vibrations are apt to be present in the stalling flow ranges with recirculation causing mechanical damage to pump and/or pumpdriver bearings.

 

[abeltio]

for centrifugal pumps at very low flow the efficiency of the pump is very low, so is the power consumption (motor amps at constant voltage).
one pump manufacturer rep explained to me that they recommend a min flow to protect the seals... for most common applications the pumped fluid is used to refrigerate the seals... with very low flow and constant speed (typical of AC motors)... as anne robinson would say: seal, you are the weakest link! goodbye.

saludos.
a.

 

[25362]

Chapter 9 of the Centrifugal Pump User's Guidebook by Sam Yedidiah (Chapman and Hall) is totally devoted to the subject of recirculation (causes and effects).

 

[JJPellin]

The statement that the low flow cavitation is a thermal affect does not seem right to me. The pump impeller will cavitate in suction recirculation mode even if the vapor pressure margin is large. I have often heard it said (and I believe it to be true, based on my experience) that is it not practical to suppress low flow recirculation cavitation with excess NPSH. It has more to do with flow, velocity and turbulence, not with simple vapor pressure margin. Some pumps (vertical turbine) can experience suction recirculation cavitation even running at BEP with a large NPSH margin if there is pre-rotation coming into the pump inlet. The cavitation does not occur because of localized hot spots, but because of localized low pressure zones in the high velocity vortices. The cavitation is a problem because it is unstable, and not occurring in all impeller vanes to the same extent. This it produces axial and radial forces at uncommon frequencies that result in shaft deflection, case vibration, coupling, bearing and seal failures.

 

[d23]

All,

There is a thermo effect when operating at low flow. Assume we had a pump that required 100 HP at an arbitrary min flow near shut-in. At this low flow the pump efficiency is only 5%; therefore we have 95% wasted energy or 95 HP that must be dissipated somewhere. A percent of this energy is dissipated in the form of vibration due to caviation that will occur at this operating point. The rest of the wasted energy must be dissipated in the form of heat.

In the metric system the formula for heat rise would be:

Centigrade rise =
((BkW – WkW) * 14.34)/((Q in Ltrs per M)* Specific Heat)

 

[JJPellin]

I agree that heat will be generated. But, as you describe, the heat is a result of the inefficiency of running at low flow. But the heat is not the cause of the cavitation as indicated in the original post. The cavitation is caused by low pressure. The heat is just an unfortunate byproduct.

 

[25362]

Past threads that may be of help:

thread407-71420
thread124-67169
thread407-105918
thread798-74894

 

[d23]

JJPellin

There is a lot of unknown data for this post. What is the NPSHa and NPSHr, how low of a flow is actually expected, what type of seal is used, what is the specific heat of the liquid being pumped, etc.

My concern with cavitation at low flow is the pump housing, shaft and impeller temperature will increase. As fluid enters the pump intake and impeller eye the fluid temperature will increase. Cavitation calculation then has to take into account the new PVT characteristics.

I know that I use the word “IF” too much, but "if" NPSHa under normal operating conditions is close to NPSHr, "if" the pump efficiency is less than 10% at the new operating point and "if" the pump is allowed to operate very long at that point you may want to reconsider the NPSHr. The required pressure at the intake will increase due to the higher fluid temperature.

Historically any restricted flow unit I’ve look at seems to have a very minor change in temperature until you get around 5 to 10 percent efficiency. At that point the temperature increases exponentially. The amount of heat is then based on the amount of HP or power that must be dissipated and the retention time. If your driver is only 10 HP you will not heat the fluid very much however; if it is a large pump that has to dissipate several hundred HP you could be well beyond vapor point at the eye of the impeller with a restricted flow.

D23

 

[JJPellin]

d23,

We seem to be talking past each other, possibly because we come from different backgrounds, disciplines and industries. I have never seen any literature to suggest a thermal affect in suction recirculation cavitation. As I stated before, I don't believe it is possible to suppress suction recirculation cavitation with excess NPSH(a). And, since I have seen suction recirculation cavitation even running at or near the best efficiency point, it is obviously not necessarily tied to an efficiency problem. I don't dispute that thermal affects can play in role in some modes of cavitation (more likely discharge recirculation), but I don't believe it plays any role in suction recirculation cavitation at low flow. My comments are based on technical literature, much of it from the Pump Users Symposium as well as 16 years of experience working on API process pumps in an oil refinery. Perhaps we can agree to disagree.

Johnny Pellin

 

[tkdhwjd]

I am not a pump expert, so pardon my ignorance if what I am addressing is incorrect. But if heat is generated, and cannot be dissipated away, will it not cause the temperature of the fluid to rise? Also, does not cavitation occur when "vapor" pockets implode as the system pressure exceeds the vapor pressure? How can you have cavitation without vapor pockets imploding? Whether it is the temperature or pressure, something must trigger vaporization of the fluid.

 

[010874]

Is there any way to handle the heat generated during the pump minimum flow bypass by ensuring a certain minimum recycle pipe length? If so, how to estimate this length?

 

[BigInch]

The pump's flow is not "mechanically" unstable. Mechanical protection refers protection of the pump by a mechanical action, ie. to the opening of a valve to allow recirculation or sensing the low flow or low pressure condition to initiate a pump shutdown.

 

[djack77494]

Thanks to all for this informative tread. As a process engineer, I've been preparing basic datasheets for pumps for many years. Typically, I've always thought of the minimum flowrate requirement as something due to the buildup of heat in the fluid and the resulting vaporization and then implosion of bubbles leading to the destructive effects. I now know that there is much more to this topic than my limited knowledge had allowed, and that other forms of failure, such as seal failures, are probably much more likely when sufficient flow through a pump is not maintained. I especially thank vanstoja for his/her aerodynamic approach to explaining the nature of the problem.

Despite my new insights, I will probably continue using my fluid properties and outdated methods to estimate a pump's minimum flow requirements. These methods are based on the problem to be avoided being fluid vaporization and subsequent collapse. Though that is not likely to be the real problem, I can generally come up with an estimated minimum flowrate and it is generally conservative.

Thanks again,
Doug

 

[BigInch]

OMG Sorry to say, especially considering your self admitted limited knowledge, ... NO, your vapor pressure tied method may not be conservative if the suction pressure is well above vapor pressure of the liquid, as is usually the case since initial design conditions will ensure that NPSH is higher than vapor pressures anyway. This can be much more pronounced in many systems, when inlet pressures are often many times higher than vapor pressures. Pipeline work for one. If you're looking at only seal temperature limits, don't go below 10% of BEP for more than a few minutes. So don't make that your minimum operating flow. It is usually the seal temperatures that will determine the allowable pump temperatures when vapor pressures are small in relation to actual inlet pressures. Other things to consider, if you are interested in system design, is that pipe coating temperatures are often exceeded before maximum seal temperatures are reached. Additionally, axial thrust pressures are often exceeded at low flows due to pressure imbalances within some types of pumps which greatly increase maintenance and do so at levels that approach 60% or less of a pump's BEP flow. See the API specs for recommended minimum flows to avoid mechanical damage. And remember that cavitation problems are also greatly increased as temperatures rise as the fluids vapor pressure increases dramatically with temperature. So, all in all, I would recommend that you do not brush off the advice you asked for so easily.

 

[BigInch]

010874

Take the heat generated by the pump and apply that to the mass of fluid contained within the recycle line circuit piping to find the increase in temperature of the recycle line contents. Assume that's equal to your pump temp.

 

[BigInch]

A bit quoted from McNalley,

"The heat generated in the pump stuffing box, between the seal faces, and other parts of the system will affect you in multiple ways. It can:

Increase the corrosion rate of any corrosive liquid.
Change critical tolerances.
Destroy some mechanical seal faces.
Shorten the life of any elastomer in the system including grease seals.
Change the state of the product you are pumping from a liquid to a gas or solid.
Increase pipe strain.
Waste valuable energy
Change the viscosity of the bearing oil and eventually cause bearing failure
On the suction side of the pump it can cause cavitation."

 

[25362]

In addition to the mentioned problems caused by internal recirculation, I wonder whether there is also a residence time consideration at low flow rates.

When speaking of flow rates significantly lower than the pump's BEP, the liquid has more (residence) time to liberate any dissolved gas when passing through zones of low pressure. Gas liberation affects the pump's performance by reducing both head and efficiency.

Since the solubility of air in some fuels is higher than in water, these effects may be more pronounced, and the NPSH requirements for air-saturated fuels may be much higher than for water.

 

[BigInch]

25362

What you mention is true and pump efficiency and NPSH available is reduced even with a small amount of entrained air, not air in solution. (And remember that NPSHR is always given for clear cold (50ºF i think) water), but residence time in the tank before it is pumped is more important, as any degassing will occur there. This is especially true of "live" oil that has been produced with some small amount of natural gas entrained in the liquid, but could also hold for water as the soluability of air in water is quite high, but that is of course in solution, not entrained. Once live oil is placed in the suction line and sent to the pump, wheather it's going to the mainline or the recirculation line, no gas is leaving the liquid when its already inside the pipe, unless you stop flow for a considerable length of time. The soluability of air and a lot of common gases is known for water, but I would be interested in finding some exact data on the maximum soluability of air in various types of fuels and other liquid hydrocarbons. Air release valves are often provided for water pipelines. I don't normally increase NPSHRs to allow for air in fuels, but I am generally maintaining rather high NPSHAs, except in the case of hot gasolines (45ºC) where it must often run right at the NPSHR limit. It does seem though that gasoline can run closer to min NPSHs without cavitating than does water, but I've no hard data to back that statement up.