3 replies to this topic

[sgoel]

We have a centrifugal air compressor for process air requirement. It is driven by a electrical motor running at 1470 RPM.

The surge control system (from CCC) for this compressor has a Inlet Guide Vane clamping based on power system frequency. The max guide vane opening is progressively reduced for reducing power grid frequency.

Can someone please provide a reason for this clamping.


Thanking you in anticipation please.


S Goel

[agorag]

Will be much better if we discuss with the performance curve of the compressor.

As frequency dips, so does the motor speed (and the output speed of the gearbox) and the compressor's ability to meet the required header pressure, especially if this compressor is working parallelly with another. Since head & flow are complementary in dynamic compressors, to make up for the head loss, CCC reduces the flow by closing the IGV. But this action causes the operating point to move closer in proximity to the control & approach lines of CCC.

However, this cannot go on. Below a certain IGV opening, the operating point's proximity to the control & approach lines on CCC would become dangerously close. CCC takes of this by clamping the IGV and prevents the compressor from surging. However, if the freq dip is more than, say 2%, CCC could be programmed to trip the compressor. Alternately, at this low frequency, the power control system itself would initiate a tripping action.

Edited by agorag, 04 May 2010 - 09:15 AM.

[sgoel]

Thank a l;ot agorag for the information.

If I understood correctly, there would be a minimum clamping on the IGV, however what we have is maximum clamping for a given frequency. The compressor curve is very steep (falls off steeply).

Thanking you in anticipation please.

Regards

Will be much better if we discuss with the performance curve of the compressor.

As frequency dips, so does the motor speed (and the output speed of the gearbox) and the compressor's ability to meet the required header pressure, especially if this compressor is working parallelly with another. Since head & flow are complementary in dynamic compressors, to make up for the head loss, CCC reduces the flow by closing the IGV. But this action causes the operating point to move closer in proximity to the control & approach lines of CCC.

However, this cannot go on. Below a certain IGV opening, the operating point's proximity to the control & approach lines on CCC would become dangerously close. CCC takes of this by clamping the IGV and prevents the compressor from surging. However, if the freq dip is more than, say 2%, CCC could be programmed to trip the compressor. Alternately, at this low frequency, the power control system itself would initiate a tripping action.

[agorag]

Your question has to be clear in the first place.

As is commonly seen, the head falls vertically after a flow, which is the stonewall end.

By affinity law, in case of compressor speed increasing with increase in grid frequency, flow, head & power increase by the 1st, 2nd & 3rd power of the speed ratio.

There could be various reasons for the IGV maximum angle clamping.
-the motor may be running close to the full load amps when on max air load operation. Any cincrease in frequency could trip the compressor, which is obviously undesirable.
-equipment downstream of thec ompressor (reactor, aftercooler, safety valve...)or other systems may have a pressure rating limitation. May be, the process itself is very sensitive to pressure & temperature variations. (as pressure increases, temperature also increases)
-the blowoff systems may have a limitation. Response delay may be more than that of power grid frequency fluctuation rate.
-blow-off noise may be a no no in terms of HSE concerns.
-or, it could, simply be to avoid stonewalling

Edited by agorag, 06 May 2010 - 10:08 PM.