3 replies to this topic

[m42364236k]

n my roject we have seal gas compressor and also gas lift compressor.
for both sealing of compressor we use discharge gas of compressor plus nitrogen.(DGS)
i have two qestion
first .if we lost compressore do we need any gas buffer drum for sealing of compressor to avoid leakage?if yes menioned buffer shall sized for how much time?
in cause and effect of licenssor docemment just for shutdown level 0 we deprresurize the compressor and for other level of shut down we have permition it meens that we can hold the compressor pressurize ,i want to know that why we should pressurize the compressor in some esd level instead of depressrizing?
 do we need buffer drum for sealing 
when  we pressurize the compressor ?

[paulhorth]

No one else has responded to this question, so I will try to give an answer.

 

I am not an expert on dry gas seals, but it is my understanding that they function as follows:

When the compressor is running, the rotating seal is held off the fixed seal face by a tiny gap, due to the action of the seal gas. The seal gas is injected inboard of the primary seal and while most of it flows into the compressor, a small flow passes out through the seal, by design, and into the cavity outboard of the primary seal, which is vented to flare or locally to atmosphere. The secondary seal prevents this gas from escaping the end of the shaft into the compressor room. Normally there is no DP across the secondary seal so there is no flow. I think nitrogen is sometimes used to purge between the primary and secondary seals.

When the compressor stops, the primary seal faces come together and form a static seal which can retain internal pressure. If I am right here, this means that a buffer supply of seal gas is NOT needed for when the compressor is shut down.

 

It is desirable on a shutdown to avoid depressuring the compressor, if it is safe, in order to allow a more rapid restart. Since the seal can hold pressure, and the cavity is vented,  there is no risk of a gas leak into the surroundings if the compressor is held under pressure. Therefore, on lower shutdown levels, blowdown of the compressor is not carried out. On the highest level of ESD shutdown, there is a major emergency (such as fire), and a restart would not be attempted, so then the casing would be depressured along with the rest of the equipment, to discharge the flammable inventory of gas.

 

Paul

[trinhduchanh]

m42364236k,

 

During compressor shutdown, Dry Gas seal must be supplied by external Nitrogent for prevent gas leakage to outside. With Ventilated shutdown after compressor stop and casing pressure arond zero then you can close the N2 SDV valve supply for Dry Gas Console. With Pressurized shutdown, you need to maintain differental pressure between N2 supply with casing pressure always above 2.5 barg.

[hamadi]

Hello

 

For compressor with ZERO pressure within the machine no need for any primary seal to be introduced, the Labyrinth between the balancing drum & seal assembly will do the job the same for the suction side.

 

When for example inerting the machine, an external primary seal should be implemented, as trinhduchanh said, use nitrogen to insure 2.5 Barg over Balancing line pressure. 

 

The seal faces on dry gas seal even in stop condition they are not and should not to be in contact , only the seal oil system faces are in contact condition especially when the machine in stop condition, in which seal oil will do the job of cooling the faces, this action is not visible in DGS . 

 

At a pressurized stop condition (depends on the design) the primary seal gas still supplied from the machine discharge and the seal gas booster will do the work to insure 2.5 Barg DP between the balancing line & the seal, be careful of the temperature of the dry gas to the seal and insure proper operation of the heater.