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Minimum Safe Flow For Centrifugal Pump




Minimum Safe Flow For Centrifugal Pump Minimum Safe Flow for pumps has been discussed on numerous occasions on "Cheresources". Many posts have discussed the line sizing, flow control and the scheme for common minimum flow piping from multiple pumps operating in parallel.

Let us discuss pump minimum flow as defined by "API STD 610 - Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries".

Minimum Continuous Stable Flow: It is the lowest flow at which the pump can operate without exceeding acceptable vibration limits.

Minimum Continuous Thermal Flow: It is the lowest flow at which the pump can operate without its operation being impaired by the temperature rise of the pumped liquid.

Pump operations below these points can cause shaft vibration and / or reduce the mechanical seal life both due to vibrations and high fluid temperatures.

To avoid such problems, high capacity pumps shall be provided with a minimum flow bypass with the flow controlled by a restriction orifice or a control valve. If a restriction orifice is used, the operating flow rate of the pump should be increased to account for the continuous bypass flow.

The sizing of the minimum bypass flow circuit shall be based on the higher of the two minimum flows.

If it is not possible to provide a minimum flow circulation line, then adequate instrument protection shall be provided to prevent pump operation below the minimum allowable flow rate.

Today's blog entry is more about "minimum continuous thermal flow". When there is no flow from the pump by way of a valve closed to the pumped liquid reciever the power input to the pump is converted to heat in the casing of the pump causing the temperature of the liquid to rise in the casing. To prevent this happening the "minimum continuous thermal flow" needs to be ensured through the pump when it is operating.

Most experienced process engineeers are aware that this minimum safe flow to prevent temperature rise is provided by the pump vendor. During the preliminary phase of engineering, when no vendor data is available for the minimum safe flow and the minimum safe flow recirculation line is to be sized an approximation of the minimum safe flow needs to be done. Often, this assumed or approximated value is 25 to 30% of the forward flow. Obviously after the receipt of the vendor data for the minimum safe flow, the recirculation line hydraulics needs to re-checked based on the value provided by the pump vendor.

However, it is possible to calculate the minimum safe thermal flow for general purpose centrifugal pumps based on the spreadsheet that I am attaching. However, this spreadsheet is only applicable for existing pumps where data such as brake horse power at shut-off, shut-off head and the pump efficiency at shut-off is available. In other words, the spreadsheet may be used to verify the minimum safe flow given by the pump vendor. The equations for calculating the "allowable temperature rise" and the "minimum safe thermal flow" are in USC units. However, I have programmed the spreadsheet to take inputs in SI units and give the output of minimum safe flow in both USC units (US gpm) and Metric units (m3/h).

Hope all of you find this blog entry and the accompanying spreadsheet interesting and useful. I would be happy to receive your comments and will try to answer any queries raised by you.

Regards,
Ankur.

Download the MS Excel Spreadsheet Here:

Minimum Safe Flow for a Centrifugal Pump




The various schemes for minimum recycle flow with their characteristics are given in my blog entry at:

 

http://www.cheresour...trifugal-pumps/

 

If a recycle line is shared by pumps operating in parallel, then it should be sized for the minimum recycle flow of one pump multiplied by the maximum number of operating pumps in the parallel pump arrangement. 

 

If individual recycle lines from each pump operating in parallel join to a common recycle header then the sizing philosophy should be as follows:

 

1. Size the individual recycle line from each pump for the minimum flow of each pump

2. Size the common header combining individual recycle lines for the combined minimum flow from all the individual recycle lines.

 

Hope this helps.

 

Regards,

Ankur.

Dear Ankur,

I wasn't able to find the attached spreadsheet. Could you please post it again?

 

Thanks,

Amit

Dear Ankur,

I wasn't able to find the attached spreadsheet. Could you please post it again?

 

Thanks,

Amit

Amit,

 

Look again at the end of the blog entry. The link is provided.

 

Regards,

Ankur.

Dear Sir,

 

Good morning

 

I am working on a project where deisobutanizer bottom pump donot have minimum circulation line, At the same time one of the DCN mentions - Min circulation is not required for deisobutanizer pump, contractor shall provide additional pump protection during detail engineering.

After a lot of discussion and brainstroming, I am not able to come to a conclusion as what can be the alternates of min circulation to protect pump against min flow.

Please give your suggestions.

 

Thanks

Jacks

Dear Sir,

 

Good morning

 

I am working on a project where deisobutanizer bottom pump donot have minimum circulation line, At the same time one of the DCN mentions - Min circulation is not required for deisobutanizer pump, contractor shall provide additional pump protection during detail engineering.

After a lot of discussion and brainstroming, I am not able to come to a conclusion as what can be the alternates of min circulation to protect pump against min flow.

Please give your suggestions.

 

Thanks

Jacks

Jacks,

 

Instrumentation alternative can be provided for ensuring that the pump does not run below the minimum safe flow.

 

A flow transmitter or flow switch in the pump discharge can be provided which can trip the pump if the flow falls below the minimum safe flow of the pump.

 

Alternatively, another scheme would be to trip the pump if the pump casing and pump bearing housing temperature rises above a permissible value. This suggestion is due to the fact that the pump casing temperature and in all probability the bearing housing temperature will tend to rise if the pump runs below the minimum safe flow for a long duration.

 

You will need to talk to the pump vendor and your instrumentation engineer to conclude which scheme suits you the best. You will also need top ascertain the SIL level for your instrumented protective function for the pump minimum safe flow.

 

Hope this helps.

 

Regards,

Ankur.

Thanks Sir,

 

It will be of great help...

 

Jacks

Dear Ankurji,

How to calculate efficeiny of pump at shut off head or else is shall be an assumption?

I am asking this since this variable has great impact on the min flow results dynamically in the spreadsheet provided by you.

Could you tell me what is gracious in a recirculation line

1. to use orifice and why

2. A control valve and why

Hi,

 

In a naphtha pump, I have come across the minimum recycle flow through restriction orifice is being cooled before sending back to the surge vessel which is under nitrogen blanketing (7 bar g). I hope this is provided to decrease the temperature rise of the fluid to protect the pump / loss of naphtha by evaporation. 

 

Please let me know what is the criteria to decide, if a cooler is required in the re circulation line ?. Is there any standard which specify the requirement ?. 

 

Thanks in advance :)

Dear Sir,

 

Good morning

 

I am working on a project where deisobutanizer bottom pump donot have minimum circulation line, At the same time one of the DCN mentions - Min circulation is not required for deisobutanizer pump, contractor shall provide additional pump protection during detail engineering.

After a lot of discussion and brainstroming, I am not able to come to a conclusion as what can be the alternates of min circulation to protect pump against min flow.

Please give your suggestions.

 

Thanks

Jacks

Hey Jack,

 

An under load trip of the motor will also solve the purpose.

 

Best regards,

Kumar

Hi,

 

In a naphtha pump, I have come across the minimum recycle flow through restriction orifice is being cooled before sending back to the surge vessel which is under nitrogen blanketing (7 bar g). I hope this is provided to decrease the temperature rise of the fluid to protect the pump / loss of naphtha by evaporation. 

 

Please let me know what is the criteria to decide, if a cooler is required in the re circulation line ?. Is there any standard which specify the requirement ?. 

 

Thanks in advance :)

Kumar,

 

Requirement of cooler depends on the storage temperature in your vessel and the temperature rise across the orifice. For sub-cooled liquid pressure drop in minimum flow restriction orifice contributes to practically no change in density and temperature, to be of any concern. I have never seen a cooler at the outlet of a restriction orifice and neither know of any installation as such. However, this does not mean that for a very specific application or design it may not be applied.

 

The philosophy of the application has to be known. One such philosophy could be that the Naphtha is at its saturation pressure corresponding to the saturation temperature when it enters the restriction orifice. As the liquid expands through the RO, it flashes at the corresponding pressure downstream of the RO. In that case, you have a drop in temperature and a two-phase liquid-vapor mixture downstream of the RO. If this 2-phase mixture is not desirable then a cooler downstream of the RO would be useful in cooling so that the exit stream from the cooler becomes a sub-cooled liquid only.

 

In one of the projects that I worked the vapor pressure of Straight-Run Naphtha from the Crude Unit was 54 kPaa (7.8 psia) corresponding to a temperature of 50°C.

 

Hope this provides some explanation.

 

Regards,

Ankur

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