4 replies to this topic

[K J]

hello every one

        In our ammonia plant reformer NG feed control valve has around 1.5 to 2% variation in controller out put and positioner feed back, it makes of 1000 to 1500NM3/Hr flow variation in 35000NM3/Hr NG feed rate and also in final syn gas flow. it has ABB make smart positioner. Design pressure drop across control valve is 2.5bar while it was remaining 2.6bar. we have recently reduced DP across valve to 2 bar but still variation is there except flow variation peaks have reduce to some extent. is there any method to evaluate performence improvement in valve after DP reduction. we have already done tuning but no good result found. variation is not cyclic.

Edited by k.j., 11 October 2013 - 06:24 PM.

[fseipel]

When you say you have tuned, do you mean you have tuned the smart positioner AND the flow controller?  I'd especially suggest tuning the smart positioner.  I would look at filter time constant on flowmeter and also look at whether valve is exhibiting sticking and whether upstream and downstream pressures are changing significantly.  I would be more interested in whether valve is at extremes of its operating range, then what pressure drop is across it, though the two are related.  What % open is the valve and what are is its characteristic (equal percentage?)

[rychurek]

We faced with the same issue when throughput was 40-50 %. Feed flow fluctuation was caused by few things : low valve opening, high dp across  feed control valve. I don't remember values because for short period of time we increased plant load and fluctuations have gone. We investigated this when compared feed flow  controller output with positioner. Controller output was increasing when valve position remained constant and after a while valve opened in one step caused feed flow increased by 1000 m3/h

[K J]

 It is koso make equal% 6" valve. tuning done on controller not on smart positioner. If filter time constant means flow transmeter damping time then it is some what 2 sec as this flow tag is going to ESD so damping is low. CV upstream down stream pressure is 32.5bar and 30.5 bar with no sudden significant change. valve opening normally remains around 60%. valve is not sowing sticking behaviour. It also has air booster, does this contribute any way.

[fseipel]

Try tuning smart positioner.  Also try putting loop in manual and increasing or decreasing valve open%/output in small increments such as 0.1% and noting if/when flow rate changes & if behavior is repeatable or shows signs of sticking/friction=stiction.  I take it by air booster, you mean that valve is fitted with a Fisher Volume Booster, VBL or similar?  If so it does have a bypass adjustment screw that might be impacting performance.  If you have AMS or diagnostic monitoring, try plotting ACTUAL steam position to see if it is fluttering.