Re: 9fa Gas Compartment Vent Fan 88vl

M

Thread Starter

Meganathan S

we are having GE 2*9fa dlan 2+ ge gas turbine. among them one unit is commissioned and now in service. we are frequently getting problems with sov's in gas compartment because of high temperature in the compartment (110c). we are suspecting our ventilation design is poor, so i need photographs of ventilation ducts arrangement, fans and vent duct lower rise photos of your sites guys...

Thanks in advance
 
Meganathan S,

The turbine and auxiliary packager (GE or BHEL or ???) should have provided a P&ID for the compartment ventilation systems, and on that drawing there are maximum values for pressure drops in duct work. Typically the drawing is called, "Flow - Heating and Ventilation." It may also show the expected heat rejection of each compartment.

Generally when this kind of problem occurs it's because the air being used to pressurize the compartment (gas valve compartments are usually pressurized--under slight positive pressure--but the compartment ventilation fan(s) to ensure that hazardous gases are being pushed out of the compartment. There have been special cases, usually because of local codes or regulations, where the compartments are under negative pressure (the fans draw air in through dampers, through the compartment, through the fan, and then discharged to atmosphere). But, if the air that's going into the compartment (either by fan pressure or fan suction) is already at elevated temperature then cooling isn't going to be very efficient.

If the compartment is under positive pressure, when someone opens a door to the compartment it will "fly open" because of the positive pressure in the compartment. If the compartment is under negative pressure, when someone attempts to open the door it will take a lot of force to open the door. The P&ID should show if the compartment should be under a positive or negative pressure; it may even list a typical pressure (positive or negative). This information should be used to determine if the compartment is under the design pressure (positive or negative).

And, the primary purpose of ventilation fans is to ensure combustible, hazardous gases don't collect in the compartment and become an explosion hazard. Cooling is important, but prevention of collection of combustible gases is usually most important. Having said that, 110C is pretty darned hot.

Are filters used to prevent ingestion of dust/contaminants into the compartment? If so, are they clean? Are they regularly inspected and replaced if necessary?

Does the unit use heated gas for performance enhancement? If so, are the gas fuel lines properly insulated? (They should be--though in some locales gas fuel flanges must be not be insulated.)

In general, inlet dampers or outlet dampers of some sort are used to block fire extinguishing agent from leaking out of the compartment should a fire be detected and extinguishing agent discharged. It's common for gravity-operated dampers to not work properly (get rusted, and fail to get lubricated regularly).

Some dampers are latched open, and the latch is operated by fire extinguishing agent pressure. It's very common for these latched dampers to be subject to vibration and for them to allow the dampers to close, preventing flow of air when the ventilation fans are running.

GE commonly provides what are called reverse tangential centrifugal fans. These fans rotate in the OPPOSITE direction of what one would normally expect a centrifugal fan to operate. When they are operated in the wrong direction (the direction they would seem to operate in) the motor driving the fan draws excessive current (more than rated nameplate). So, it's important that the fan be rotating in the proper direction--and that direction should be painted on the fan enclosure by the manufacturer, or, more ideally, it should be a stainless steel tag with an arrow in the direction of rotation that is riveted to the fan enclosure (NOT the motor housing).

Checking motor current draw is a good way to see if sufficient air is moving. If the current draw is much less than motor nameplate then it's a safe bet the fan isn't moving as much air as it's rated to move. If the motor is drawing much more current than nameplate then either the motor is running in the wrong direction or there's a drag on the fan (bad bearings; rub; etc.). But, the motor should be drawing something near nameplate current if it's working properly.

A lot of time duct work is added to the inlet- and outlet of the fan. Frequently this ductwork is improperly constructed, resulting in a higher than expected pressure drop which results in lower than required air flow. Again, the P&ID should detail the maximum expected pressure drops across inlet- and outlet ducts.

Sites are all different, meaning that some turbines are not inside a building and some are. Those that are inside a building usually have ducts for inlet and exhaust from each compartment, and it's important that the pressure drops be less then cited on the P&ID. And, that inlets and outlets not be blocked, get cool, clean air for the inlet and exhaust to atmosphere with as little back-pressure as possible. If filters are used, they need to be regularly inspected, and cleaned or replaced as necessary. If heated fuel is used, then the fuel lines should be properly insulated (per local regulations, as appropriate).

Again, 110C is darned hot for a gas valve compartment--particularly one that doesn't use heated fuel. Please ensure that the compartment is properly sealed, per the P&Id, and not subject to ingress of hot gas from the turbine compartment or some other source.

Please write back to let us know what you find--and how you resolve the problem.
 
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