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Selecting the right mechanical seal system for rotating equipment involves factoring in both equipment operating conditions as well as any environmental restrictions. Where toxic substances are concerned, and no leakage or evaporation is permissible, a clean barrier fluid is needed to flush the seal and prevent any egress of the pumped liquid.
Toxic chemicals cannot be blocked by single seals; but require a dual sealing system. The American Petroleum Institute’s (API) Standard 682 specifies requirements for mechanical seals, described as Plans. API Plan 53 (ANSI Plan 7353) lays out three methods (A, B and C) to eliminate any possibility of leakage or emission to the atmosphere.
Each of the three methods makes use of a pressurized external fluid reservoir and circulates a lubricating barrier liquid through the seal, isolating the pumped product from the atmosphere and extending seal life by providing a proper lubricant rather than letting the abrasive or non-lubricating pumped fluid damage the seal. Unlike other plans, Plan 53B requires an accumulator installed on the sealing circuitry in order to keep the pressure on the seals.
Plan 53A has no moving parts but relies on a gas, typically nitrogen, to maintain pressure on the barrier fluid. Although this is the simplest type of Plan 53 design, it is not recommended for use above 150 psig since the gas can become absorbed into the pumped fluid at higher pressures.
Most complex of the three, Plan 53C, uses a hydraulic cylinder to maintain the barrier fluid at a higher pressure than the pumped liquid. The sealed piston is pressurized by the pumped fluid with an input/output ratio typically about 10% higher than the system pressure.
These systems usually include an internal cooling coil or an external cooler. Between these two very different approaches lies Plan 53B, which uses a pre-pressurized bladder accumulator to maintain the barrier fluid pressure (Figure 1).
Plan 53B is designed to eliminate the pressure limitations associated with Plan 53A and is used for applications or products that have high pressure and are harmful or hazardous. In the Plan 53B seal support system, a clean fluid film is maintained between the seal faces using a barrier fluid that is pressurized by an inert gas.
Any heat in the barrier fluid can be removed by an air-cooled or water-cooled heat exchanger. A pumping ring circulates a barrier fluid through a loop that includes a seal cooler and other instrumentation. Excessive seal leakage is detected by a drop in the pressure in the seal loop.
The gas and barrier fluid are kept separate through a bladder in the accumulator. The bladder is pressurized with nitrogen prior to filling the system with barrier liquid. As the system is filled, the bladder is compressed by barrier liquid. The over-pressurized nitrogen provides constant pressure on the barrier liquid.
Unlike the 53C design which maintains a constant pressure ratio between the barrierfluid in the pumped product, the 53B system maintains a constant pressure on the barrier fluid regardless of the pressure of the pumped liquid. This means that the outboard seal is subject to the maximum pressure at all times, while the inner seal faces a variable differential pressure as the pump pressure changes.
With a 53B design, a separate cooler is recommended. Because the separate heat exchanger introduces additional flow resistance to the piping system, this may affect the circulation of the barrier fluid. The 53B skid specification must take this into account to ensure adequate flow.
Fluid Energy Controls manufactures Plan 53B accumulators for safe handling of a variety of harmful or toxic chemicals. Because the barrier fluid is kept at a higher pressure, a small amount will leak through the seal and into the process fluid.
The barrier fluid, therefore, must be selected to safely interact with the process fluid, and plan 53B equipment must be specified to safely handle that barrier fluid. Normally these accumulators use stainless steel or nickel-plated carbon steel accumulators with a capacity of 2.5 to 10 gallons. Bladders inside the accumulator shells can be made with Buna-N or Viton elastomers. Viton is more resilient at higher temperatures.
The accumulators can include any fluid port from NPT, SAE O-ring boss, or Code 61 flange. These systems typically keep the pressure on the seal at about 35 psi higher than the process pressure, ensuring that no process fluid leaks through the seal. Since the pressure on the seal is maintained by the pressure of the nitrogen gas in the accumulator, temperature fluctuation of the system may also cause pressure fluctuation on the seal. The accumulator can be equipped with a pressure switch and a temperature monitor in order to regulate and stabilize the pressure in the bladder, and hence, on the seal.