Ron Astall of United Pumps Australia dives back into the world of pump curves to explain how understanding seal chamber pressure could leave you flush with success.
Often, seal and pump personnel are unclear about each other’s expectations. In the previous article, I looked at the pressures inside pumps and how these pressures affect seals. We also covered basic principles, examining pump internal construction and discussing how the pressure is distributed within the pump casing. The focus was on single-stage end-suction pumps, but the logic extrapolates to other configurations. Most single-stage conventional pumps will have front rings, back rings and balance holes. For these, the seal chamber will be at or slightly above suction pressure. For smaller pumps, there may not be back rings, and the seal chamber will be closer to discharge pressure.
In that previous piece, we asked, “Will flush piping change the pressure in the seal chamber?” The short answer is that it may. Seal piping systems are usually classified as either primary or secondary. Primary seal piping affects the seal chamber conditions inboard of the mechanical seal (or gland packing). Secondary systems are for dual seals and regions outboard of the seals. For now, we will only discuss what we call primary seal piping. That’s because this directly affects the seal chamber conditions, which dictate the rating and type of seal selected.
A flush creates flow
Fundamentally, a seal chamber flush creates a flow of liquid. A flush may also alter the seal chamber pressure, but only if the flow is restricted.
A tight clearance or an orifice in the flush circuit will create a change in pressure, depending on whether you are upstream or downstream of the restriction.
Why have a seal flush?
API 681 / ISO10349 standards define standardised piping plan layouts, which are useful across all pump types and applications. A selection of common primary piping plans and their expected impact on seal chamber conditions is shown here:
| Common primary flush piping plans – what do they achieve? | |||||||
| Flush Plan | |||||||
| Effect on Seal Chamber | 01 | 11 | 13 | 23 | 31 | 32 | |
| Maintain Stable Liquid Film | √ | √ | √ | √ | √ | √ | |
| Ensure Complete Venting | √** | √** | √ | √** | √** | ||
| Provide Cooling Flow | √ | √ | √ | √√ | √ | √ | |
| Boost Pressure | √** | √** | √** | √** | |||
| Reduce Pressure | √ | ||||||
| Provide Clean environment | √ | √ | |||||
| Avoid Difficult Liquid Properties | √ ? | √ ? | √ | ||||
**depending on seal chamber design.
Let’s look at several of these flush plans in more detail.
Plan 01 and Plan11
Plan 01 and Plan 11 are the most common flush plans. For many people, they are interchangeable as they create a flow from a high-pressure region to the seal chamber. Usually, the high-pressure region is the pump discharge; for multistage pumps, it may be from an intermediate stage. Plan 01 (Figure 1) uses internal passages in the pump casing, and the flow does not enter through the seal plate. For this reason, Plan 01 is not as flexible as Plan 11 (Figures 2 and 3) in controlling pressure and heat removal.
For Plan 11, the use of external piping allows the use of an orifice to control flow. The flush past the seal faces limits local heat build-up but does not provide additional cooling. Because of the direct connection through the seal plate, the flush can also be directed more accurately to the seal faces.
When used in conjunction with a Throat Bush, Plan 11 can also increase seal chamber pressure.
Plan 13
Generally used for vertical pumps, Plan 13 (Figure 4) has the primary advantage of ensuring positive venting of the seal chamber. As with Plan 11, flow past the seal faces limits heat buildup but does not provide additional cooling. Flow is from the seal chamber back to pump suction, which means that the seal chamber needs to be above suction pressure to ensure adequate flow.
As with Plan 11, an orifice is usually installed to control the flow. This arrangement is often used when the pump does not have back rings and balance holes, particularly when the seal vendor wants a pressure higher than pump suction pressure in the chamber for vapour suppression purposes. When a throat bush is installed in the seal chamber, the Plan 13 bleed back to suction can, conversely, be used to reduce seal chamber pressure.
Why have a seal flush?
A flush is often indicated to improve the environment for the mechanical seal. We need a stable liquid film at the seal faces. An appropriate seal flush plan can ensure venting, cooling flow, and pressure control.
This article has only briefly scratched the surface showing several simple primary flush plans for single seals. We have also seen how a throat bush in the seal chamber can assist with pressure management.
In the next instalment, we will discuss more of these plans and how to more positively manage liquid properties at the seal through cooling, solids removal, and clean injection.
