Mechanical Seal 2

[mucour]

We have a centrifugal pump in our facility using flush Plan 11. Liquid is taken from the pump discharge and connected through an orifice into the mechanical seal at the top flush connection. The Flush (F) connection location is at 0 degrees while the Drain (D) connection location is at 180 degrees.

But I noticed that there was nothing coming out of the drain piping connected to the mechanical seal bottom.

Could anyone explain to me where the liquid flowing into the mechanical seal is disappearing? Is it expected to flow out through the drain point or is it going back into the pump through the throat bushing?

Any help will be appreciated

 

[daveboy]

Mucour

Not sure on your actually system or seal but usually with a plan 11 the flush fluid passes through the seal and goes back into the pump through the throat bush.

The drain connections are generally used with a quench connection so if you don't have any quench connections then you are unlikley to get any fluid out the drain.

David

 

[mucour]

We use John Crane single cartridge mechanical seal.

In one of the facilities, liquid is seen draining out of the mechanical seal through the drain connection port. But in another facility, nothing was coming out of the mechanical seal. Both facilities are installed with pumps using Plan 11.

What do this two situations suggest.

 

[pmover]

is pump operating? developing head?

are you certain the flush tubing/piping is not filled with debris? new installation or existing?

what is pumpage (fluid being pumped)?

is fluid flashing across orifice? process parameters?

has seal recently failed?

i'm certain the situation is simple to explain.

good luck!
-pmover

 

[jet1749]

Have you got the connections mixed up? You say you have the flush port connected to the pump discharge (which should then pass liquor back into the pump) but the flush connection should be at 90 degrees to the quench or drain connection not 180 degrees. As mentioned previously any connection to quench should allow flow out through drain or vice-versa. Have you checked the orifice is clear? If your other facility has the seal connected the same and liquor is passing through the drain connection it would indicate that the mechanical seal is leaking.

 

[checman]

Mucour: I agree with daveboy as far as the flush, quench and drain connection. It sounds like the seal may be leaking at your other facility. Or they might have the flush line connected to the quench port which then the flush fluid would leak out the drain.

Regards checman

 

[JJPellin]

Unfortunately, the seal manufacturer's are not all consistent in the use of names for the various connections. A drain connection can sometimes be a process side connection that might be used to drain the seal chamber when the pump is taken out of service. In other cases, drain would refer to an atmosphere side connection that is between the mechanical seal and the bushing (somethimes called the disaster bushing). With John Crane, it is most likely that the drain connection is atmosphere side and any fluid coming out of that connection indicates that the seal is leaking and should be replaced. The seal flush in a Plan 11 passes across the seal and into the process steam through the throat bushing. I am not aware of any fixed standard for the angular possition between two connections. Any connection could be oriented in any direction depending on your piping requirements and machinery design. So having the flush on top and the drain on bottom does not sound unusual at all.

 

[mucour]

Thank you for your responses.

JJPellin, if mechanical seals are constructed in line with APIstd682, then the convention is 180deg between the flush and drain ports, I stand to be corrected.

Our pump is a multistage pump (7-stage)and the pumpage is crude oil with sizable content of sand. The pump impeller is in between bearings. There are mechanical seals on both ends.

What I find interesting was that the flush piping was connected from the first stage discharge and then piped to both ends of the mechanical seals.

Questions:
1) Could the flush liquid from the first stage discharge flush effectively the seal on the 7th stage end of the pump?

2) Could pumpage flow from the 7th stage through the throat bush since the pressure at the 7th stage will be greater than the 1st stage discharge pressure?

Your thoughts will be appreciated.

 

[checman]

mucour: The pumps that I am familiar with that are similar to the one that you describe have a balance drum right after the last stage impeller. On the outer side of the drum is a balance line that goes to the suction side of the pump or back to the supply. In either case the pressure after the drum is approximately suction pressure so first stage discharge pressure works well to create the required flow.

Regards checman

 

[JJPellin]

Checman is correct. Any 7 stage pump of this type that I have ever seen had a balance line and both seals should be exposed to a pressure only slightly above suction pressure. I had not realized about the API-682 requirement for angle of connection. We generally do not adhere to 682 with out new seals. And the vast majority of our seals were designed long before 682 existed. And even with 682, the configuration of the pump would trump. Sometimes there is a slot through the bearing bracket at the top to allow for a top connection into the seal gland. And sometimes there is not. If there is not a slot, I would have to move that connection to the side so I could get the piping out. 682 also includes stipulations that the process side connections and atmosphere side connections are different sizes. I don't recall which ones are 1/2" NPT and which ones are 3/4" NPT. This also reduces confusion and the chance of accidentally hooking one up to the other.

 

[longeron]

The connections should also be marked... "F" for flush, "D" for drain, "V" for vent.

Viscous fluids like crude oil are more difficult than they might seem at first glance. Sure there is great lubricity, but there is also a lot of heat generated at the seal faces due to viscous shear. A crude pump that I'm familiar with operates at ambient, but the seals consistently run 60 F hotter than the process fluid because of this. The viscosity also tends to make the faces run further apart, so leakage is increased and more often than not- visible. Try a steam quench on the atmospheric side of the seal to help heat things up and drop the viscosity of the fluid at the faces. Hard faces are a must due to the solids, of course.

Also note that if there is siginificant wear of the bushings/ balance drum between the seal chamber on the high pressure end of the pump and the impeller, the pressure will increase... so when you replace the seals take a look at the bushings.

 

[mucour]

Thank you guys.

JJPellin: We use API682 to complement our inhouse standard. API682 requires 3/4" NPT minimum for the process side and 3/8" NPT for the atmospheric side. Thanks again

 

[mucour]

I have read somewhere that for multistage pumps, the flush line should be connected from somewhere in between the stages.

As the two ends of the seal chambers are connected together through a blancing line, couldn't there be possibility that the pressure in the seal chamber close to the 1st stage suction be more than the 1st stage discharge pressure. This could justify the need to connect the flush line somewhere in between the stages.

Though our multistage pump has the flush line connected to the 1st stage discharge, what is your experience in this area.

 

[JJPellin]

The configuration that I am most familiar with would have one seal (usually the inboard) directly adjacent to the first stage impeller eye. This seal will see suction pressure and can never see more that that. The seal on the opposite end will see a pressure that is suction pressure plus any pressure drop in the balance line. As the balance piston clearance opens up, the flow through the balance line will increase, and so will the seal chamber pressure at the other seal. It is possible that the pressure on this second seal could eventually get higher than first stage discharge. But by this time, the balance piston clearance is opened up so much that the pump is at risk of a catastrophic thrust bearing failure and should be overhauled. On a seven stage pump, running the flush from first stage discharge seems like a good choice to me. It could be run from second stage discharge, but this will require smaller orifices in the flush lines which are then more likely to plug off. This is especially true in Crude service. I would stick with the configuration as-built.