9 replies to this topic

[sudheer]

Greetings. 

In one of vacuum distillation units, bottom pumps are facing serious pumping issues.  The bottom pump discharge pressure slowly comes down from rated value of 35kg/cm2 to 15kg/cm2. in a matter of 15 minutes. The pump is being operated above minimum required flow.  We have checked suction pressure and found to be above NPSH required. By increasing quench, bottom temperature maintained at 330°C, the phenomenon is repeating.  In fact keeping higher level in the column is also of no use.  Suction line was water flushed by shutting down column, strainers were checked, to find no choking in suction line. Level indicator calibration also showed no improvement. Request forum to throw more ideas so that proper solution can be reached.

Thanks

[PingPong]

The bottom pump discharge pressure slowly comes down from rated value of 35kg/cm2 to 15kg/cm2. in a matter of 15 minutes.

When did, or does, this happen? Did it happen once and since then the pump only delivers 15 kg/cm²? Or does this happen often, so the pump runs at 35, or 15 kg/cm², or something in between, every day?

 

Does the problem also occur when the spare pump is used?

 

How old is the unit? In other words: how long has the pump operated without this problem?

What happened before the problem started? What was changed in the operation of the unit before the problem started? Other feedstock? Other heater outlet temperature? Other column pressure? Or did the problem start after a startup from a maintenance shutdown? Or what?

 

How is the quench mixed with the liquid from the last tray? Is there a mixing tray? Where is the TC element located?

Can you upload the P&ID showing column bottoms, quench TC, bottoms pump?

Edited by PingPong, 18 August 2014 - 10:44 AM.

[Art Montemayor]

“Throwing" ideas on this topic would be just that: Throwing them without rational or logical bases.  You should supply detailed, complete, and specific basic data on this application:

• How many pumps are you writing about?  You use the plural “pumps” and then you switch to the singular tense, “pump”.  Which is it??  If there is more than one pump, state that fact and support it with a detailed sketch diagram showing the piping and controls of the entire system – suction and discharge.
• Is this a NEW problem?  If the pump(s) system has satisfactorily worked in the past, then so state it.  If it has ALWAYS worked incorrectly as described, how has this occurred and why has it been tolerated?
• WHAT type and kind of pump(s) are you writing about?  I presume the pump(s) are centrifugals – but are they multi-stage?  - are they regenerative turbine type?  Please furnish the performance curve, complete with any NPSHA calculations available.
• You state: “The bottom pump discharge pressure slowly comes down from rated value of 35 kg/cm² to 15 kg/cm²”.  Pumps don’t generate pressure, they generate fluid flow and fluid friction or throttling on the discharge system is what determines the pump’s discharge pressure at the discharge flange.  Where are you measuring the discharge pressure and how?  What flow resistance is the pump(s) pumping fluid against?
• Have you disassembled the pump(s) and confirmed that all internals are according to specifications? 
• What fluid(s) are you specifically pumping with the pump(s)?  Is it clean and what is its viscosity?
• The suction pressure is not enough information to determine if the NPSHA is more than the NPSHR.  How have you measured the NPSHA and where are your calculations?

​As you can see, more specific information is needed.

[Padmakar Katre]

Hi,

Few points from my end,

1. How is the level monitored, DCS and thru field operator too?

2. Column bottom temperature profile over a span which includes normal pump operation and offset conditions too. Is there any pattern?

3. Suction valve is motor operated or is it gear/manual operated? in case of MOV; was there a history of malfunction? Is it possible to keep motor DE-energized after suction valve is fully open.

4. How is the quench flow controlled (bottom temp as master and quench flow as slave?) ensure column bottom temperature and quench flow instruments are intact and well calibrated, DCS numbers are in line with field values.

5. How is the minimum circulation system of pump i.e. automatic or manual?

6. How is the downstream system, is the destination pressure for Vacuum residue constant or varies? Share Vacuum residue circuit with details along the destination details (may be delayed Coker, rundown tanks wherever)

[breizh]

Hi ,

What about the level of the column  ? Which technology ? Verified , calibrated ?

my 2 cents

Breizh

[sudheer]

Greetings... Let me give complete detail of the system.  There are four multistage, backpull type, centrifugal pumps, two of which are high capacity type (35kg/cm2 pressure and 210 cu.m/hr flow) and other two are low capacity ( 18kg/cm2 pressure and 29 cu.m/hr flow).  The pumps are handling short residue or Vacuum residue from bottom of vacuum column. Depending on quantity of residue generated pumps will be operational. Typically the fluid temperature is at 350°C. Never high capacity pump will be operated in parallel to small capacity pump.  Whenever flow is beyond 29 cu.m/hr, high capacity pump will be operated with minimum circulation flow control valve open to safeguard the pump.  The pump that has been discussed in the problem are both high capacity pumps

The problem of drop in discharge pressure is random in nature with no connection bottom temperature, level in the column, downstream process conditions.  

The unit is barely two years old and the pumps have not been checked so far at full load as feed was not available.

No change in heater outlet temperature, flash zone temperature of column, stripping steam flow, column bottom temperature are observed during the start of problem

Quench is mixed in the last tray and the bottom temperature is cascaded to quench flow.  Temperature is measured in the column bottom line.

Pump suction pressure is around 350mmHg when measured at 1m elevation from pump eye.  NPSH available as per datasheet is 4.1m, elevation diffference between column bottom to pump is around 7m. Suction line pressure drop has been calculated and found minimum.

There has been vibration in the discharge line, seem to be originating from pump when pump is losing head.

We are yet to open the pump for checking.

Individual suction of pumps are manual operated valves. But common MOV at column bottom is open and kept locked.

Minimum circulation through the control valves.

Vacuum residue pumped from these pumps passes through a series of exchangers and through flow control valve goes to battery limit.  At battery limit there are two destinations, one is with back pressure controller to tank and other is with flow control either to internal fuel oil or coker unit.

There are five level indicators at column bottom three of which are DP type and two are Remote seal type.

Hope I have answered all the querries.

Thank you

Sudheer

[Ajay S. Satpute]

^Sudheer,

 

^{1. Re-do the NPSHa calculations to be sure that it is not the cause for pump losing head.}

^{2. It is assumed that there is no vapor in liquid at pump suction. Can one challenge this assumption?}

^{3. Can the total flow through pump be measured? If yes, is it matching the pump curve?}

^{4. The flow too away from POR (preferred operating region) may cause reduction in head and pump vibration. Is it possible here?}

^{5. The minimum recycle line control valve, is it passing more flow than it should (valve passing)?}

^{6. Check the pump datasheet and compare the operating conditions considered then, are still being followed?}

 

^{A rough sketch would have been great.}

 

^Regards.

 

^{Ajay S. Satpute}

[PingPong]

Maybe the minimum flow recycle of the pump is not large enough to avoid cavitation.

 

Can you post the vendor pump curves, including that for required NPSH?

 

Can you post your calculation of available NPSH?

 

Where exactly is that troublesome pressure measured? Immediately at the pump discharge (before any control valve), or further downstream?

 

Can you post the DCS screen printouts and P&ID's from column bottom part (incl. mixing tray) up to the column level control valve(s), and including quench recycle back to column?

[bhatya77]

Sudheer,

 

I agree with Mr PingPong. As the pumps are for higher capacity, if the minimum circulation line is undersized or control valve in return line malfunctions, there will be cavitation which justifies vibration  and loss of pressure. As in your case if requirement increases beyond 29 m3/hr, directly 210 m3/hr capacity pump starts which is almost 10 time higher capacity so definitely this may be the case. Providing VFD will solve your problem as it will vary rpm according to your requirement which will avoid cavitation

[Zubair Exclaim]

you say there is vibration also .. one cause is usually when flow is dropping below MCSF... but during this time the discharge pressure should give you shut off head you need to check that 

 

... you need to recheck minimum flow line ....your turn down is too huge ... if you have  a bypass to minimum flow reciculation valve, you can do a test run by opening it with the min recirc valve ... and try operating pump some where close to BEP with a higher  flow this will confirm shut off case...

 

other obvious case could be cavitation