API Seal flush plan 32

[zipped]

Hi Frnds
In case of pumps with API seal flush plan 32, what should be the seal flush pressure.
how much more than suction pressure?

 

[JJPellin]

You did not provide enough information to answer your question.

We normally adjust API Plan 32 flush for a target flow rate which is defined by the seal manufacturer for the particular service. A rule-of-thumb for 3600 rpm pumps would be to inject 1 gpm for each inch of seal diameter. But, since Plan 32 is arguably the most expensive flush plan, there is a value in limiting the flush flow to the lowest value required for good seal reliability.

If you are forced to adjust the flush flow based only on pressure, you need to adjust it relative to seal chamber pressure, not suction pressure. You need to take into account the configuration of the seal chamber, including the design of the throat bushing.

If I was forced to adjust the seal flush with no other information, I would perform the following: Block in the seal flush and read the seal chamber pressure with no net flow in or out. Open up the seal flush until the seal chamber pressure increased by at least 10 psi. Many people would use a higher value. Some sources suggest 25 psi above seal chamber pressure. This would probably result in excessive flush flow. But, at least you would know that you were getting a good flow.


Johnny Pellin

 

[bk19702]

JJPellin is right on with the adjustment for flow based strictly on pressure. One item that needs to be considered is why the Plan 32 is being used to begin with, which is typically because the pumped product is either not a good lubricant or something that you want to keep away from the seal components altogether. In this case, you size your Plan 32 flow based on throat bushing velocity. A general rule of thumb is 15 ft/s across the throat bushing to keep solids, etc. excluded from the seal face area. You will need to know your throat bushing clearance, length, and adjust your plan 32 source pressure accordingly to achieve both the flow across the seal and velocity across the bushing that is required. In reality, your actual flush flow rate to cool the seal may be much less than the 1GPM / inch rule, but you need to evaluate what is occurring across the throat bushing as well. If you don't already have one, a close-clearance throat bushing may need to be installed in the seal chamber as typically an OEM throat bushing may have a more generous clearance than what is required for your purposes. How you plan on controlling the 32 flow rate also requires discussion - there are different methods depending on what you are trying to achieve. As mentioned, more information is required for an accurate assessment.

 

[zipped]

Thanx JJ & bk
We are using seal flush 32 in combination with plan 52.
Pump service is HD Polyethylene slurry. Hexane is used as a flushing media in plan 32 and White oil in plan 52.
A throat bushing is installed in the seal.
Now my doubt is:: With the same throat bushing, can we vary (increase) the flow rates from seal to the pump casing. Actually our process guys needs more hexane from seal flush plan 32 going to the impeller. This is to minimize sticky polymer depositions on the impeller.
If yes, does increased flow rate affects the seal life / performance.

 

[JJPellin]

That's a new problem that I have not come across before. You need to inject too much flush in your Plan 32 to meet some other process need. Injecting too much seal flush can have a couple of detrimental effects. And, you still don’t give enough details to evaluate either one. Excessive seal flush can increase the pressure in the seal chamber. This could reduce seal reliability. This depends on the supply pressure of the flush, the throat bushing geometry, the maximum design pressure of the seal (bellows or pusher) and the fluid properties of the flush. Excessive seal flush can result in jetting of the flush disturbing the seal faces. This depends on the flush injection point (head or gland), the injection configuration (single point, multi-port), face materials and the properties of the flush fluid.

You are only giving us tiny bites of information to evaluate. If the process requires injection of additional flush beyond the requirements of the seal, I would suggest having the process engineers find a different place to inject it. Using the seal as an injection point does not seem wise given the limited information provided.

We once had a bright, young process engineer use the high point on the balance line of a large multi-stage pumps to inject a chemical for activation of the unit catalyst. The pump failed in thrust which required a very expensive, unplanned overhaul. I would suggest you be very careful about using seal flush for a process need.


Johnny Pellin

 

[bk19702]

Using the mechanical seal as a process injection point could be a bad idea for the reasons mentioned above. You need to evaluate the desired expectations for the injection against the capabilities of the seal. It would be in your best interest to sit down with your process people, mechanical seal manufacturer, and possibly the pump OEM and describe what you are trying to accomplish.