Plan 11 is the most common flush plan in use today. This flush plan simply takes an appropriate amount of fluid from the discharge of the pump (or the discharge of one of the intermediate stages if applicable) and puts it into the seal chamber to provide cooling and lubrication to the seal faces.
- No product contamination. The flush source is coming from the pump and going back to the pump.
- No reproduing of product. Unlike an external flush, the product does not have to be reprocessed.
- Simplified piping. Piping consists of only pipe (or tubing) and an orifice, if required.
- With a properly sized orifice and throat bushing that results in a higher seal chamber pressure, the vapor pressure margin can be increased.
- If the product in the pump is not a good face lubricant or is dirty, the seal can become damaged or clogged.
- Flush has to be re-pumped. Circulation from the discharge back to the pump suction will decrease pump efficiency and increase power required for the application. Usually the volume of flush is very small compared to the capacity of the pump and therefore the efficiency effect is very small.
Generally the flush rate must be calculated based on service conditions, pump speed and seal size. The rule of thumb is for not less than 1-gpm per inch (0.16-l/m per mm) of seal size, but the flush requirement may be greater if the pressure or speed is high. For application above 3600-rpm or box pressures above 500-psig (35- barg) the flush rate should be calculated to avoid excessive heat at the seal.
The flush flow rate is usually controlled by an orifice in the flush line. Orifices should not be less than 1/8-in (3-mm) unless the product is very clean and customer approval is obtained. Many small or low speed pumps have a low differential pressure and no orifice is required in the piping.
An interesting challenge arises when the differential pressure is high and a 1/8-in orifice allows for more flow than is desired. This can be addressed two ways. One option is to use two or more orifices in series. The number is dependent on the differential pressure. The other way is to use a "choke tube". This is a piece of tubing generally ¼-in heavy wall. The length of the tubing is calculated using a piping pressure drop calculation such that the pressure drop across the tubing is equal to the difference between the discharge pressure and the seal chamber pressure at the flow rate desired.
Any flush system works hand in hand with the hardware and seal parts. If the seal is set up with a distributed or extended flush, the effectiveness of the system will be better and the seal will run cooler no matter how much or little the flush flow rate.
Pumps & Systems, May 2007