6 replies to this topic

[siraj paracha]

We have a BFW pump installed which is a steam turbine driven. Maximum Capacity of pump is 380 m3/hr. Discharge pressure of pump is 146 kg/cm2. 

 

Since long we are encountering a high pressure drop problem in BFW header. On checking it was observed that the spool from pump immediate discharge to shutoff valve upstream has a pressure drop of 9.5 kg/cm2. This spool contains a Minimum Flow device designed for 85 m3/hr flow in shutoff position, a NRV  & 03 90 deg. Elbows. Schematic is attached along with pressure measurements for reference.

 

My question is about the pressure drop in this spool piece, like is it normal for BFW pumps immediate discharge spool to give this high amount of pressure drop. what part of the spool is playing its part in high pressure drop?

 

If it is MFD which is causing this drop in pressure, do any one have idea about MFDs which are designed to give low pressure drop in line?

 

How can we overcome this high pressure drop problem ?

 

Your help is required.

 

Thanks

 

 

 

 

 

Attached Files

•  BFW pressure profile - Comparison with digital PI.pdf   322.19KB   44 downloads

[Mahdi1980]

Siraj,

 

Please show the minimum flow device on your sketch.

Normally, MFD or Restriction Orifices are located on a side branch to connect the discharge and suction of the pump avoiding temperature increase through the pump and they do not cause such a dramatic pressure drop within the main line or header.

Moreover, what I see in your sketch is that the pressure difference between your local PI is 2 bar while that of digital ones are 9.5 bar.

So, it means that there is a fault in your digital PIs.

Have you checked them work correctly? Have you calibrated them?

 

Mahdi

Edited by Mahdi1980, 30 May 2016 - 07:03 AM.

[siraj paracha]

Dear Mahdi,

 

I have cross checked my Digital PI with the transmitter installed on the line as you can see on the sketch both showing the same pressures. Second i have calibrated the Digital PI on the calibration bench as well.

 

Now about the MFD is yes it is installed at the side branch of the pump discharge line and circulates back the BFW to De-aerator.

 

I have also considered the passing of MFD which may cause the pressure drop at the discharge line, I have measured the temperaturs across MFD line but temperatures are not that much high which reflects passing through it.

 

Now the question arises that if MFD do not add its part in pressure drop than what could be the possible reason of such a high pressure drop?

 

Thanks for the support.

[samayaraj]

Hi siraj,

 

Have you checked the flow through MFD during normal operation? In my plant we faced the same as the MFD/ Automatic recirculation valve malfunctioned and there was a continuous flow through it thought the pump is operating at rated flow. This caused more load on the boiler feed pump motor and reduction in delivery pressure.

 

I suspect the issue is with MFD and it suppose to open at closed discharge/ reduced flow condition.

Edited by samayaraj, 30 May 2016 - 11:17 PM.

[Mahdi1980]

Siraj,

 

It is not clear for me. In your P&ID sketch , the local PI just at discharge of the pump shows 148 kg/cm2 and that one at the end of spool piece shows 146 kg/cm2. Is it true? ( of course, I can see the pressure transmitters show the high pressure drop. however, the local PI shows just 2 kg/cm2 pressure drop according to your data.)

 

If it is just a mistake in writing, Okey we should find the solution.

So, please let me know what is the length of the spool piece ( distance between the two PIs) and the bore size of it and discharge line?

Are you sure that there is no blockage in the spool piece? Have you opened it yet to check ?

If MFD is located on the side branch, it is not the cause of the pressure drop of the header. You should check the spool piece.

 

Regards

Mahdi

Edited by Mahdi1980, 31 May 2016 - 02:44 AM.

[S.AHMAD]

SiraJ,

1. Are you experiencing same high pressure drop when running the spare pump P4601B?

2. Normally the spool piece has NRV and isolating vale, correct?

3. If the problem is only with P-A then most likely the check valve or the isolating valve partially stuck at small opening.

4. Minimum flow if designed and installed correctly is not relevant to the high pressure drop.

 

S.ahmad

[siraj paracha]

S. Ahmad

 

Both pumps are designed for 340 m3 with 40 m3 extraction from 3rd stage. Turbine driven normally remains in service while motor driven is on stand by. When turbine driven is in service we pump around 375 to 390 m3 flow through it while Motor driven pumps 340 m3 flow at final disharge.

 

As motor driven supplies less flow normally that is why we use it along with BFW import from parallel plant, but we face pressure drop problem with that pump as well.

 

Thanks