By Ron Astall FIEAust CPEng NER RPEV, United Pumps Australia
This article explores the pressures inside pumps and how these pressures act on the seals.
Often seal people and pump people are not clear about each other’s expectations. For mechanical seal reliability, we need to understand the conditions in the seal chamber. The starting point: what is the pressure in the seal chamber?
To demonstrate basic principles, for now, we will discuss single stage end suction centrifugal pumps.
Aren’t seals always at suction pressure?
The most common assumption is that the mechanical seal (or gland packing) will see suction pressure. In the absence of any other data, that’s usually a good assumption, but not always.
Figure 1 shows a pair of single stage end suction overhung pumps. They look similar; both pumps have close clearance sealing rings between the impeller and casing. The pump on the top has these close clearance sealing rings on both sides of the impeller – called front and back rings. The pump on the bottom only has front rings. The pump on the right also has ‘Balance Holes’ through the impeller hub.
How do these differences affect seal chamber pressure? Let’s look at each design in turn.
Pumps without back rings
The typical pressure distribution is shown in Figure 2. Because there are no impeller/case rings at the non-suction (back) side of the impeller, discharge pressure will fill the entire casing. Due to centrifugal whirling effects the pressure at the seal chamber will be slightly less than discharge pressure. If the actual seal chamber pressure cannot be measured, a good rule of thumb is to use suction pressure plus 75 per cent of the differential pressure.
This design is common for smaller pumps and avoids the internal recirculation path of back ring leakage. The downside is that higher pressures on the back side of the impeller create an axial loading on the pump bearings. This thrust issue is the reason larger pumps will have both front and back rings with impeller balance holes.
For difficult sealing conditions, however, construction without back rings may be deliberately used to increase the seal chamber pressure. On end suction pumps, this construction (without back rings) may also be used deliberately to create an opposing axial load when there is a high suction pressure pushing the shaft in the other direction.
A common variation is the semi-open impeller design, where the front shroud of the impeller is open, and the impeller vanes run closely against the casing to form a dynamic seal. Semi-open impellers also commonly have clearing vanes at the rear of the impeller. Clearing vanes will reduce the pressure at the seal chamber significantly and will also reduce the axial thrust. See Figure 3.
The pump shown here has full diameter clearing vanes. This will theoretically reduce the pressure at the seal chamber to suction pressure. As wear occurs and the
gap between the rear of the impeller and the back cover increases, the pressure reduction will become less effective but still significant.
Smaller, partial clearing vanes will provide a smaller reduction in seal chamber pressure. The only certain way to know the exact seal chamber pressure where clearing vanes are used will be by measurement during normal operation. Nevertheless, the pressure will be certainly less than discharge pressure. The worst-case assumption would be to use the values as per Figure 2.
Pumps with front rings, back rings and balance holes
For medium to large pumps, this is the most common construction. The typical pressure distribution is shown in Figure 4. The balance holes in the impeller hub theoretically equalises the pressure on both sides of the impeller, whilst allowing the back ring leakage flow to return to suction. Hence the seal chamber will be at (or just slightly above) suction pressure.
For difficult, engineered applications the pump designer may deliberately reduce the size of the balance holes to provide a slightly elevated seal chamber pressure. Normally, we can assume we are close to or slightly above suction pressure with this configuration.
What’s the pressure in the seal chamber?
We’ve covered the basic principles – now we need to look at a pump’s internal construction and understand how the pressure is distributed within the pump casing. We have focussed on single stage end suction pumps, but the logic can be extended to other configurations.
As a starting point, most single stage conventional pumps, usually 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. Remember to use the full operating range of differential heads from the pump curve for your pressure calculations. For semi-open impellers and impellers fitted with back clearing vanes, the advice of the pump manufacturer should be sought if accurate values are needed.
The above logic is a good starting point. Things become more interesting when we have more adventurous applications involving difficult liquid properties. It may be necessary to manage the conditions in the seal chamber to ensure seal reliability. Then we will need to consider the role and the effects of seal flush systems and auxiliary piping plans.
Effects of seal flush piping
Will flush piping change the pressure in the seal chamber? It might indeed.
That’s a topic for another time.
Featured image: Part of an industrial pump sealing system. mage: nordroden/stock.adobe.com
