Intermediate Blow Off valve on Axial Flow Compressor

[Poovan]

We have recently experienced severe damage on an axial air compressor. The damage was caused due to multiple interstage surges. The direct cause of this was intermitent opening of the intermediate blow off valve. The valve is situated after the first intercooler, before the air is fed into the second compression stage. Can anybody shed some light on the purpose for such a valve at this location. Our investigations have shown that almost everytime this valve opens, there will be localised surges on the second compressoin stage.

Please help.

 

[yates]

Blow-off valves are designed to avert compressor surge by dumping excessive pressure to atmosphere usually during the speed-up stage of the compressor. If the opening of the blow-off valves are causing surge, then there would appear to be a problem in the valve regulation loop, possibly the valve is oscillating due to a lack of correct damping factor in the loop.
Has this been happening since the compressor was new ? since a recent overhaul ? what is driving the compressor, a gas turbine ? Is the valve opening with subsequent surge occurring during run-up or at normal operating speed ?

 

[hacksaw]

what is the current control scheme for the blow-off valve

 

[Poovan]

The fault occured during normal operating conditions. There was a fault in the control loop that triggered the valve opening unneccessarily. We have identified that much. The problem is that the intermediate valve was install by the compressor supplier as a modification to the initial design.

This valve actually vents the air from the first compression stage to atmosphere! During normal operation, this suffocates the second compression stage and hence causes surges on the second stage.

This modification was done a few years ago but nobody here seems to understand the reason for this particular valve. We have 2 other blow off valves on the compressor dischargs (after the third compression stage)that perform the normal function of anti surge valves.

Can you provide information/insight about the need for the intermediate blow off valve?

 

[hacksaw]

The uncontrolled release of pressure at 2nd stage suction while the machine is at normal operating condition, will cause severe surge no question.

Having a valve there is okay, but it has to have quite sophisticated controls and interlocks. How complex? Depends on the machine.

Purpose? Only the manufacturer has the engineering files. Possibly an attempt to avoid surge during a start up case.
It should have been locked closed.

Was the valve included in the HAZOP?

 

[rlewis]

I can only speculate that this may have been added to unload the unit during startup. I can't imagine using it during operation.

 

[1969grad]

As I recall intermediate stage blowoff valves are used to balance the flow to the next stage in partial load conditions. The manufacturer would match the wheels for each stage at full load but the flow at lower speeds do not necessarily match the wheel characteristics of each stage.