This article is related to an earlier Sealing Sense that answered the question "When and how do I use API Flush Plan 53?" (June 2005). Now we will focus on the three variances of Plan 53 (see Figure 1).
Plan 53 is the most commonly used auxiliary system for the operation of dual pressurized mechanical seals. It provides a clean external barrier fluid at a constant or variable pressure greater than that of the product pressure on the inner seal to ensure virtually zero emissions to the atmosphere. The mechanical seal will usually contain an internal pumping ring or screw that circulates the barrier fluid through some type of cooling device, which is installed in close proximity to the pump.
Figure 1. Overview of API Plan 53A, 53B and 53 C
From an operational point of view, the system must maintain pressure above product pressure at all times.
It must also maintain the barrier fluid within a specific temperature range such that the seal faces operate under favorable lubricating conditions, typically below 150 deg F for most barrier fluids. The saying "a cool seal is a happy seal" is true in most applications and should be considered during the selection and sizing process of a dual mechanical seal support system.
From a maintenance point of view, the system must allow for replenishment of fresh barrier fluid without interruption of the operational requirements. The amount of clean, fresh barrier fluid makeup depends on the leak rate of the seal faces. The refill frequency can be estimated or predicted by comparing the available volume of the reservoir with the anticipated leakage of the seal over time. Contact a seal OEM for estimated leak rates since they allow the use of barrier fluid consumption as a benchmark for the performance of the seal and predict the need for maintenance.
All three plans require some type of makeup system for barrier fluid lost through natural leakage of the mechanical seal. The makeup system can be a simple hand pump (see Figure 2) or a more elaborate system that automatically feeds multiple seals.
The choice of the barrier fluid is the most important consideration for determining the best system. Water-based fluids will dissipate heat twice as effectively as petroleum-based fluids and leak rates may be much different for the same operating conditions. The important point is that a water-based fluid, i.e. 100 percent water or glycol/water mixtures, may be used in a smaller system as compared to oil-based fluids, all other operating conditions being equal.
Basis for Selection
Plan 53A is the simplest plan of the three; it has no moving parts and is easy to operate. A gas, usually nitrogen, is used to maintain constant pressure on the barrier fluid in a stainless steel reservoir. The gas is typically sourced from a plant system. API 682 specifies these systems should not be used above 150 psig because of the danger that gas absorption in the barrier fluid may affect the lubricating state or regime of the sealing faces.
The reservoir has typical fluid storing capacities between 1 and 5 gallons. Cooling is obtained by circulating the barrier fluid over a cooling coil in the reservoir. Cooling water flow rates through the coil will vary between 1 and 3 gpm for the majority of applications. Cooling capacity is typically limited to approximately 6 kW for the larger reservoir sizes. The signal for barrier fluid refilling is normally delivered by a level switch in the reservoir or by visual monitoring of a level gauge.
This system is commonly used on single overhung pumps and only one dual seal can be operated with each system. A variety of highly standardized products is readily available for ANSI and API applications from most seal OEMs.
Figure 2. Plan 53A hand pump for barrier fluid
In Plan53B, the pressurizing gas does not come into direct contact with the barrier fluid. Instead of the storage and cooling reservoir in Plan53A, a heat exchanger (air or water-cooled) maintains suitable temperatures for the barrier fluid. A pre-charged bladder-type accumulator maintains reasonably constant barrier pressure. A bottle of nitrogen is used to pre-charge the accumulator and a hand pump brings the loop to the desired barrier pressure.
Since a plant nitrogen system is not required, this plan is suitable for remote installations where no or limited utilities are available. From a pressure range standpoint, Plan 53B is typically used for pressures between 150 and 750 psig. The size of the accumulator is critical since sufficient volume compensation must be available to com pensate for the barrier fluid lost from normal leakage of the mechanical seal. Volume expansion of the barrier fluid must be included in the selection and sizing process. The refill signal is created by a pressure switch that senses barrier pressure decay and notifies the need for refill well before the product pressure is reached. Periodic checking of the accumulator pre-charge is necessary and manual refilling can be used if the anticipated leak rates are reasonable.