DGS Buffer Gas Flow Rate is Too Low

Though not familiar with this type of seal, I ask out of curiosity: Is there really a problem with low buffer gas flow? (There could conceivably be a cooling function, but that doesn't seem likely at the low flow.)

Please keep us posted on what the seal mfr says.

what is the leak off flow rate to flare?

it is normal for the seal gas flow to drop with no load to full load as the reference / seal chamber pressure increases (against which the seal seals). But what we have normally seen is that the flow stays higher than normal during no load and comes to normal flow when the compressor is loaded.

is your seal gas flow filter DP normal? High DP may cause low flow

are your flow measurements ok? meters calibrated?

Finally when the Seals were installed, were the Rotors positioned properly in their correct running positions, if not we may see such low flows as the seal faces seperation could have been affected due to improper positioning of Rotor.

Thanks Mr. gsbasha for your repaly,

leak off rate is about 10kh/hr (normal is 5-6kg/hr)

seal gas flow filter DP is normal

flow measurement is ok it was checked

thanks again,

Do you have a secondary seal gas injection for your containment seal? do you monitor the secondary seal gas flow?

when your primary seal gas flow is only 3.6kg/hr then the leak off to flare should be less than this if there is no secondary seal gas injected in to your seals.

carry out mass flow balance with Primary seal gas supply, Primary seal vent to flare, secondary seal gas supply, secondary seal vent to flare, seperation seal gas supply etc. to doubly confirm that the flow measurements are ok, or provide all these values so as to understand further and hopefully guide you to right solution. Also include the seal drawing.

Why John Crane? This is to be clarified by OEM of compressor, M/s MAN Turbo. What are the seal clearances specified by OEM, have you confirmed it while replacing it? What are the ranges of seal gas flow rates for each stage specified in O & M manual from OEM?

A little more information!! Gas handled? Dischare pressures? Capacity? Is it four pinion shafts driven by a single bull gear?

Please specify which clearance do you mean? it is cartridge, 28 type.
Gas Handle is CO, Discharge pressure is 60bar, 7th stage located in HP compressor which has two pinion shaft one bull gear.

Thanks in advance

As always more inforation is required. I am familiar with gear compressors but not certain of the Man Turbo design. You indicate a 7 stage compressor. Typically on a gear compressor there are 1 or 2 stages for each pinion. So when you say there are 7 stages and 2 pinions, it is not adding up.

You are talking about flow and control valve, but there are different ways to control flow, so without a P&ID it is difficult to identify with the change in a pressure controller how this would affect the flow to the seal.

With an overhung type compressor, which a gear compressor falls under, the actual seal pressure is related to the drop in discharge pressure across the back of the wheel. So the sealing pressure is highly dependant on operating discharge pressure and design of clearances around the compressor wheel.

To identify your problem the discharge pressures and temperatures are required, an understanding of the control system that is being used and the design parameters from the wheel and surrounding components that may effect the sealing pressure. It also has not been identified, but what design of seal is being used? Single, tandem, tandem with intermediate or doble seal? my assumptions is this is a single seal, but not certain. A trend of operating conditions of the compressor and operating conditions of the seal(s) is needed. Did the seals fail that were replace? If so what was the failure mode of the seals? There are probably many more questions that will need answers before a conclusion can be established for your problem, but this would help provide a truer picture of what might be your problem.