Centrifugal pump idling cooling water flow calculation

[quizzical1]

Hi All,

NFPA 20 4.13.1.4 states “The valve shall provide sufficient water flow to prevent the pump from overheating when operating with no discharge”.
Does anyone know where I can find the equations or an online calculator to quantify the required flow for keeping an idling centrifugal pump cool?

thanks,

Q~

 

[EdStainless]

Many pump curves/data sheets give a min flow cut off.
It is usually 20-30% of BEP depending on the pump style.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[1503-44]

You're better off getting a minimum flow recommendation from your pump manufacturer.

Minimum flow requirement varies with power, discharge temperature, recycle fluid temperature, pump discharge pressure and efficiency.
It also depends on the return temperature of the recycled liquid, a tank to draw or mix cool water with the recycle flow can be helpful, if the pump keeps heating up the return flow recycled temperature
For now you might estimate it at 20% of the pump's rated flow.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

These are fire pumps remember.

They used to test for 30 mins with a closed in head....



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[shvet]

Does anyone know where I can find the equations or an online calculator to quantify the required flow for keeping an idling centrifugal pump cool?

Only a combination of particular installation conditions and particular manufacturer is able to provide you with this info as overheating is a too complex thing. In cases such kind of experience is the best advisor.

INFLUENCE OF PUMP HYDRAULIC SELECTION
The major components of the life cycle cost of ownership are initial cost, installation cost, operating cost, and maintenance cost. In process plants it has been found that under many circumstances the cost of unscheduled maintenance is the most significant cost of ownership. Other chapters of this book deal primarily with mechanical means of improving reliability, while this chapter addresses pump life improvements that can be achieved through proper pump hydraulic selection. Further improvement could be realized if more attention were paid to key hydraulic factors, such as those associated with pump hydraulic forces (dynamic and static), cavitation, and wear within the pump (Ref. 5-1).
Improvements in Mean Time Between Failure will be limited in potential unless a holistic approach is used. Such an approach gives more attention to the best hydraulic fit to optimize reliability. The authors believe that there are four basic hydraulic selection factors that can have a significant effect on pump reliability. These are pump speed, percent of best efficiency flow, suction energy and NPSH margin ratio. These last two factors have further been combined into an NPSH margin reliability factor (NPSHRF), which has been shown to be reasonably effective in predicting the reliability of high suction energy pumps.
The “Mean Time Between Repair” (MTBR) and Life Cycle Cost of most centrifugal pumps can be improved if slower pump speeds are used, and pumps are generally selected to operate in their preferred operating range (70% - 120% of BEP flow rate, Refs. 5-2 and 5-3). Further, the mean time between repair of high and very high suction energy pumps can be increased by keeping the NPSH margin ratio above certain minimum levels and/or by reducing the suction energy level. The easiest way to lower the suction energy and increase the NPSH margin of a pump application is by lowering the speed of the pump. This chapter deals with these issues. Laboratory tests on three API end suction pumps, and maintenance data collected on 119 ANSI and split case pumps were used to validate the hydraulic selection reliability indicators.

OPERATING SPEED
Operating speed affects reliability through rubbing contact, such as seal faces, reduced bearing life through increased cycling, lubricant degradation and reduced viscosity due to increased temperature, and wetted component wear due to abrasives in the pumpage. Operating speed also increases the energy level of the pump, which can lead to cavitation damage. Figure 5-1 compares the API-610 pump laboratory reliability predictor test results with the reliability trend line from actual MTBR data on 119 actual process pumps, as a function of the ratio of the actual to maximum rated pump speed (Ref. 5-3). The reliability factor for the field test data was based on zero pump repairs in a 48-month period, which was assumed to be equal to an MTBR of 72 months. Both curves show a marked increase in reliability with reduced speed.

ALLOWABLE OPERATING REGION
Design characteristics for both performance and service life are optimized around a rate of flow designated as the best efficiency point (BEP). At BEP, the hydraulic efficiency is maximum, and the liquid enters the impeller vanes, casing diffuser (discharge nozzle) or vaned diffuser in a shockless manner. Flow through the impeller and diffuser vanes (if so equipped) is uniform and free of separation, and is well controlled. The flow remains well controlled within a range of rates of flow designed as the preferred operating region (POR). Within this region, the service life of the pump will not be significantly affected by the hydraulic loads, vibration, or flow separation. The POR for most centrifugal pumps is between 70% and 120% of BEP.
A wider operating range is termed the allowable operating region (AOR). The AOR is that range of rates of flow recommended by the pump manufacturer over which the service life of a pump is not seriously compromised (Ref. 5-4). Service life within the AOR may be lower than within the POR. To use an analogy: While it may be possible to drive a standard automobile in first gear at speeds in excess of 25 mph (40 km/h), doing so for a long time will come at a price.
The following factors determine the AOR, with the degree of importance dependent on the pump type and specific design:
• Temperature rise
• Bearing life
• Shaft seal life
• Vibration
• Noise
• Internal mechanical contact
• Shaft fatigue failure
• Horsepower limit
• Liquid velocity in casing throat
• Thrust reversal in impeller
• NPSH margin
• Slope of the head-rate-of-flow curve
• Suction recirculation (Ref. 5-5)
The flow ratio (actual flow rate divided by BEP flow rate) affects reliability through the turbulence that is created in the casing and impeller as the pump is operated away from the best efficiency flow rate. As a result, hydraulic loads, which are transmitted to the shaft and bearings, increase and become unsteady. Also, the severity of these unsteady and often oscillating loads can reduce bearing and mechanical seal life.
Operation at reduced flow rates that put the pump into its recirculation mode can also lead to cavitation damage in high suction energy pumps. Field and laboratory reliability data are presented in Figure 5-2, as a function of the flow ratio. Correlation between the field and laboratory data is good in the normal operating range, with the maximum reliability existing around 90 percent of the best efficiency flow rate.

 

[quizzical1]

thanks, guys - i really appreciate all the help!

 

[Artisi]

Why are you asking, is this a NFPA installation and are you using a NFPA approved pump set?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[quizzical1]

No, not an NFPA installation - I was just quoting from their Standard and it made me wonder if there were any online calculators that could estimate the minimum continuous thermal flow for a given pump setup.

Here's what i found:

https://www.easycalculation.com/physics/fluid-mechanics/centifugal-pump-minimum-continuous-flow.php

Seems convenient, but fully realize the specific pump manufacturer's recommended minimum flow data would take precedence as others on here recommended.

 

[LittleInch]

Love to know what their actual "calculation" is....

considering that it doesn't ask for the pump rated flow or BEP flow or head or anything it's a very odd looking page.

Also odd mixture of units.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

Couldn't be bothered wasting time on it, but looks like nonsense to me.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[pierreick]

Hi,
Let you study the documents attached.
EDIT:

https://ekwestrel.com/what-is-the-purpose-of-minimum-flow/?utm_source=EmpoweringPumps

Pierre

 

[TenPenny]

If you want to get heavily into it, you could figure out what the BHP (kW) load at zero flow is.

Then, assume that energy is going into heating the liquid.

Find out the volume in the casing, then use the kW load to figure out the temperature rise in the liquid.
And then from there figure out the required flow to prevent overheating.


Or, simply use the minimum flow figure provided by the manufacturer; yes, this is generally a hydraulic/mechanical minimum flow based on shaft loads and bearing life, but it would certainly be a conservative figure, unless you're already pumping very hot water.

 

[quizzical1]

Thanks again everyone. I really appreciate the help!

 

[georgeverghese]

Usually, the recirc rate required to keep a pump from overheating is less than 3% of that at BEP, while min flow to keep pump flow stable and for mechanical integrity is in the range 25-30% of that at BEP.

 

[quizzical1]

georgeverghese, that's exactly what i'm looking for. can you cite any references?

 

[LittleInch]

I don't think you'll find a written reference that you can quote. That (25-30% of BEP) is just a ROT based on the decades of experience seen by the persons responding.

This is in lieu of data or a first pass until you get the definitive pump curve from your vendor.

IME it is rare to see it <20% or >40% BEP, but each vendor takes its own view and particular to the pump design and power.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Error in my previous post:
"Usually, the recirc rate required to keep a pump from overheating is less than 3% of that at BEP"
What I meant to say :
At equilibrium, the fresh feed rate required to keep a pump from overheating is less than 3% of that at BEP.

This fresh feed rate requirement can be calculated rather easily from first principles.
Energy into pump circuit must be dissipated by net feed flow through pump. So if the max permissible op temp is say T, feed temp is t1, then, at close to dead head and equilibrium,
P = m.Cp.(T-t1),
where P = pumping power in kW, m = fresh feed rate in kg/sec, Cp is in kJ/kg/degC
If the recycle stream goes back to suction source drum, it will take longer to get to T than it would if the recycle stream goes back into the suction line of the pump.

As stated by @LI / @Ed and others, the min flow for hydraulic - mechanical stability is roughly 30% of flow at BEP at any given speed.

 

[shvet]

@quizzical1

270 Maintaining Acceptable Flow Rates for Centrifugal Pumps
271 General
All centrifugal pumps operate best when flow rate exceeds 40–50% of best efficiency flow. Deviation from this range can cause heat buildup, excessive vibration, damage, and failure. Figure 200-48 shows this operating range (minimum flow and maximum flow). Operation at or below minimum flow is especially critical for high speed pumps (such as Sundynes) because vibration can quickly cause gearbox damage.

Refer to Standard Drawing (GA-G1097-2) in the standard drawing section of this manual to determine the recommended minimum flows for specific pump selections. This drawing will usually be conservative. Several methods of pump control are used to prevent pump operation outside a preferred range: pressure control, flow control, and less commonly, electronic control based on electric power consumption. See Figure 200-49 on the page following and Figure 200-50 for schematics.

Pump controls circulate fluid from pump discharge back to a suction vessel or tank to maintain a minimum flow rate, or they impose backpressure on a pump to prevent runout. Runout is defined as operating beyond a pump's maximum recommended flow rate. Runout is most likely to be a problem when discharge lines are short (no friction loss) or when pumping into a system with low backpressure (e.g., an empty tank).
Operation of a centrifugal pump against a closed block valve can cause overheating, vibration, and eventual pump failure, and should be avoided for any significant length of time. It is normal operating procedure, however, to start centrifugal pumps with the discharge block valve “cracked open” (i.e. nearly closed) and the suction valve wide open. The discharge valve should then be gradually opened as discharge pressure increases. This promotes quick build-up of pressure and prevents cavitation, which can cause pump failures.
When using a recirculation bypass, never return fluid directly back to pump suction—this will cause swirling and heating problems which may raise vapor pressure and affect NPSHA. Instead, route the bypass line back to a tank, vessel, or heat exchanger.

More see

https://www.eng-tips.com/viewthread.cfm?qid=501898

Important note is that pump Vendors refer to Best Efficiency Point and Rated Point while Designers refer to Required Point. This may have a significant impact of final system design. See more details concerning "rated vs required" in API 610.

 

[shvet]

Pumps in clean liquid services shall be suitable for continuous operation at 30% of the rated capacity given on the pump data sheet. The contractor shall advise the Owner if any service requires a circulation system for continuous operation at the 30% level. Turndown for pumps in slurry service shall be reviewed and approved by the Owner prior to pump selection. The 30% figure may, of course, be modified for the specific project, but experience shows that this is a reasonable minimum to specify generally without requiring the installation of large numbers of minimum flow recirculation systems.

Section 5.0
... Normally all pumps shall be suitable for continuous operation at a flow of 30 % of normal capacity. If flow conditions necessitate flow of less than 30 % minimum special flow provisions may be required. Minimum flow provision for centrifugal pumps shall consist of a line from the pump discharge to the suction source (vessel), or to the suction line through a cooler. Minimum flow required shall be based on the pump manufacturer's recommendations, but 30 % of normal flow may be used for initial minimum flow line sizing.

2.2.3 MINIMUM FLOW
A minimum flow shall always be maintained in a centrifugal pump. This may require a manual or automatic valve (restriction orifice, valve or "schroeder" check valve).
The minimum is specified by the pump supplier. It is usually 20 to 30% of the nominal flow, except for high-speed pumps, for which performance curves are bell-shaped and the minimum flow can be up to 50% of normal flow.

4.1.8 Pump Minimum Flow
The estimates of pump minimum flow provided by suppliers are too often incorrect, and lead to issues during operation. The pump minimum flow can be estimated according to the method published by Warren Fraser in his paper “Flow Recirculation in Centrifugal Pumps” presented at the 10th Texas A&M Turbomachinery Symposium in 1981.

 

[quizzical1]

thanks everyone!