API Plan 72/76 Secondary DGS Failure.

[RogerSAM]

Hello,
I just graduated 6 months ago and have started working in a fertilizers plant producing Ammonia for the further end product manufacturing of Urea. We have a vertical VS7 pump with the service of Ammonia and it has a dual (front-to-back) seal from Flowserve (type: QBQW / GSL) with nitrogen as the buffer for the secondary GSL seal.
The problem is frequent failure of the secondary with MTBF of ~6months. The API plans being utilized are 72/76. There are similar pumps installed but they are utilizing John Crane seals that are slightly different and are running fine. A few anomalies I have observed are:
1. The orifice at the CSV has a very small hole of 0.8mm dia. The similar, albeit larger capacity, pumps have the buffer outlet orifice of 3mm. Is that something of consideration?
2. Although the plan describes an orifice installed at the buffer inlet, I have noticed there is no orifice installed in field. Should that be affecting the separation of the faces of the secondary seal?
3. Before the startup of the pump, when nitrogen is being supplied to the GBI, the pressure drop between inlet and outlet is negligible. Note that the PIs are installed just before GBI for inlet and just upstream of the PI and downstream of the CSV. In other similar pumps, considerable pressure drop is noticed before the startup. Does this indicate that in the problematic pump, the nitrogen is being held in the buffer gas chamber? if so, shouldn't it be beneficial for the secondary seal, then why is the secondary seal failing so often with wear marks on the seal faces? If that is how it should be, why is there a considerable pressure drop in non-problematic pumps. Is it because of the larger orifice size and so buffer gas is not being contained in the chamber, and then is that contributing to the secondary seal failure? How is buffer gas not being contained better for the non-contacting secondary seal?

Is there any similar issue in your knowledge which could provide some guidance to me?

Thank you for the help.

 

[SB1976]

Hi Roger,

Regarding your questions, please note :

(1) The orifice at the CSV is incorrectly sized, for which the back-pressure on the sealing system is higher than required leading to gas accumulation and eventually over-pressurization of the sealing chamber. Orifice size is a function of the across pressure differential. It is possible that even with larger capacity of similar service pumps but with same suction / discharge pressures, the sealing chamber is same in both cases implying no significant difference in orifice sizing.

(2) If there is no orifice installed at the buffer inlet, how do you ensure correct circulation flow to the seal as well as seal chamber pressure maintenance? In the event of a reverse flow or partial loss in flow, the separation distance increases leading to loss in sealing capability. Strongly recommend to rectify.

(3) I understand when you said orifice is not installed in field, you mean that for the problematic pump and NOT for the other pumps (where it is indeed installed). If that is the case, then it explains all : A pressure drop in buffer gas clearly indicates that orifice is working. By virtue of its installation, it is regulating the flow causing pressure drop.

The secondary seal is failing because the pressure differential (del-P) across the secondary seals are very high. In case of primary seal, the pressure differential is between sealing pressure and buffer pressure while in case of secondary seals, it is between atmospheric and buffer pressure.

Trust above helps.