Ammonia seal leakage.

[shyamshins]

Dear Sir,

I have ammonia seal leakage issue in fertiliser plant. Pump on which seal is installed is a Ammonia booster pump.

This is double seal with seal face combination of SIC V/S SIC in Inboard side & SIC V/S Carbon at outboard side.This seal has common gland which incorporates both Inboard & Outboard stationary face of SIC. Stationary & Rotary O rings are of EPT.

OBSERVATION OF FAILED SEAL :
Inboard failure observation:
Stationary SIC face found cracked in pieces & rotary SIC face cracked in pieces.
EPT O rings of both rotary & stationary face shape changed to sqaure.
Stationary SIC face has deep groove mark on its contact face.

Outboard failure observation :
Stationary SIC face has deep groove marks on its contact surface which indicates insufficient lubrication & rise in tempreture, but barrier fluid is provided for lubrication.O/B rotary carbon face is found scored.
Rotary & stationary O rings shape changed to square shape.

Process Details are as below :
Process Liquid : Ammonia
Suction pressure : 17 Kg/cm2
Discharge pressure : 20 Kg/cm2
Process Temp.: 15 to 35 Deg.celcius.
Runout of pump : 0.02mm
Axial / Radial play : 0.01mm / 0.1mm

These pumps have 3mm hole at 12 O clock position for venting in stuffing box.
1.Flushing plan 11 & 53.
2.Tempreture at gland plate connection at barrier inlet to seal is 45Deg celcius & temp. at Barrier outlet from seal to thermosyphon pot is 47.6 Deg celcius.
3.Temp. at thermosyphon pot cooling coils inlet temp.( From seal oulet ) is 39.5Deg.celcius & temp. at thermosyphon pot cooling coil outlet ( towards inlet of seal ) is 40.4 deg.celcius.
4. Vapour pressure of Ammonia at 100deg. farenhite is 200 Psig.
5. Vapour pressure of Methanol (Barrier Fluid) is 100 mm of Hg / 0.133bar @ 21.2 Deg.celcius
6. Viscosity of Methanol at 100 F = 0.5 Mpas
7. Viscosity of Ammonia at 100 F = 0.01044 Cp

Pls. provide solution as this seal is failing every 4 months due to which we are not getting seal life.

 

[georgeverghese]

Is this a fixed speed pump or VSD? If pump speed turns down low often, barrier fluid recirculation rate may not be enough with inbuilt pumping ring.

 

[shyamshins]

Sir this is a fixed speed running pump with 2800rpm.
Can there be any other cause of seal failure as erery 4 months O rings change there shape to square which main failure reason.

 

[shyamshins]

Also for I/B faces plan 11 is incorporated. Inspite of using EPT O rings seal fails. We have alsi tried with kalrez O ring even kalrez O ring changes shape to square.

 

[georgeverghese]

Is the seal pot a bladder type accumulator or a plain pot with N2 blanketing on top of methanol barrier fluid ?

Am not a seal expert - my understanding of a double seal with flush plan 53 is that both primary and secondary seal faces are fed with the barrier fluid for cooling and lubrication. And in this case am assuming both inboard and outboard shaft seals are plan 53 flushed. So what is the plan 11 doing here ?

 

[shyamshins]

Seal pot contains methanol as barrier fluid & is pressurized by nitrogen 2kg/cm2 above stuffing box pressure. Syphon pot also has water inlet & outlet for cooling barrier fluid.

Plan 11 is provided for flushing primary seal from its O.D. Ammonia will come from plan 11 flush the primary seal on its O.D. & will go in stuffing box of pump.

 

[shyamshins]

Sir in this type of sealewe can see O/B seal with our eyes & is on atmospheric side of seal.

 

[georgeverghese]

Okay, so only the secondary seal face of both inboard and outboard seals is plan 53 flushed with methanol barrier fluid.

Since you have N2 blanketing, there should not be any corrosion due to oxidised MeOH. But check where you got this methanol from when you filled the seal pot - was it originally stored under N2 blanket?

At 17barg conditions in the primary seal, sat temp for ammonia is 45degC, so the ammonia is still in liquid phase when max discharge temp is only 35degC.

But how do you ALWAYS maintain discharge pressure of 20barg? If the discharge pressure some times drops low, plan 11 cooling flow on primary seal face wont work due to lack of sufficent differential pressure ?

 

[shyamshins]

Both Faces of primary as well as secondary seal is flushed with MEthanolfrom its I.D.
This is booster pump which increases pressure of ammonia.

 

[georgeverghese]

Loss of ammonia flushing can also be due to high suction pressure ( >17barg) at the time when discharge pressure is 20barg.

Maybe check with the pump and seal suppliers whether MEG (mono ethylene glycol) can be used as the barrier fluid here for plan 53 - I dont see why we need to use methanol, since this is in pressurised, ambient temp operations on ammonia.

Maybe some others can also help out here..

 

[shyamshins]

Sir can anyone help me to resolve this seal failure issue.

 

[flexibox]

It sounds like you have reverse pressure issues. In order to establish the root cause you should include a P&ID of your system. How do you top up the system? You shouldn't need plan 11 with 53. Why do you have plan 11?

Can you attached a seal drawing as well? I may be able to give you some information..

 

[shyamshins]

Dear all,

Yet I am not able to upload seal drwg.
Can suction press of 17 kg/cm2 & 20 kg/cm2 discharge press. cause high vapour pressure margin ? or vaporisation of product?

In this seal I/B seal stationary SIC face is always found grooved & carbon face nose is weared out.

I/B stationary face EPT O ring is eaten away which I feel is because of seal faces heat generation or might be due to insufficient lubrication through plan 11. I/ B rotary face O ring is always found extruded.

Is API PLAN 52 suitable for this type of application?

 

[shyamshins]

This is a booster pump meant only for boosting pressure of ammonia from 17kg/cm2 to 20 kg/ cm2.

 

[georgeverghese]

Can you pls advise if suction pressure is always at 17barg and does not fluctuate upward at any time ?

Also is the discharge pressure always >20barg ?