Pumps & Systems, February 2013
When set up correctly, a Plan 53B pressurized seal system has the ability to reliably deliver pressurized barrier fluid to the mechanical seal, dissipate the heat load absorbed into the barrier fluid from the mechanical seal and provide an early indication of deterioration in the seal’s performance.
Plan 53B differs from Plan 53A in two main areas:
- Barrier fluid circulating loop—All the barrier fluid stored in a Plan 53A system flows through the mechanical seal barrier fluid cavity. However, in a Plan 53B, only a portion of the total barrier fluid volume flows through the barrier cavity. The remainder is stored in the accumulator, ready to enter the circulation loop and replace any fluid lost through leakage that normally occurs with mechanical seals.
- Method of pressurizing the barrier fluid—Unlike Plan 53A, which uses an external pressure source to pressurize the fluid in a pressure vessel, Plan 53B generates pressure by pumping barrier fluid into a hydraulic accumulator, where it squeezes a gas-charged rubber bladder. The advantage of this plan is that the pressurized gas is separated from the barrier fluid by the rubber bladder, eliminating issues with solubility of gas into the barrier fluid. No external utilities are required to maintain the pressure.
The pressure generated by the accumulator is determined by the initial pressure (pre-charge) of the gas-charged rubber bladder. As barrier fluid is pumped into the accumulator, the pressure increases from the initial precharge pressure. Any temperature changes to the gas stored in the accumulator caused by variations in ambient temperatures can cause changes in the pressure. The pressure at any point can be described by the combined gas law:
P = Pressure (in absolute pressure units of measure)
T = Temperature (in Kelvin or Rankin)
V = Volume of gas in the accumulator bladder (liquid volume = total accumulator volume less gas volume)
Subscript 1 represents the conditions when the bladder was pre-charged.
Subscript 2 represents the conditions being evaluated.
|Figure 1. Plan 53A (right) and 53B (left)|
The accumulator size is selected to provide sufficient barrier fluid working volume (the volume of barrier fluid consumed by normal leakage over a period of time, typically one month) together with a small safety reserve volume and a reasonable range in barrier pressure as the working volume is consumed.
This pressure variation is determined by the ratio of liquid to gas in the accumulator. The lower the ratio of liquid to gas, the smaller the pressure variation and the more reliably the mechanical seal will perform. Typically, 15 to 25 percent of the accumulator total volume is barrier fluid. Consideration must also be given to the compatibility of the bladder material with the selected barrier fluid.
One of the main advantages of Plan 53B is the flexibility offered in choosing the heat exchanger style and capacity. The heat exchanger is no longer confined within the space of a vessel as in traditional Plan 53A systems. This allows heat exchangers of larger cooling capacity to be selected, making Plan 53B well suited to higher temperature applications.
When matching the heat exchanger cooling capacity to the heat load placed on the barrier fluid, consider the effect of increases in the resistance to barrier fluid flow commonly associated with larger heat exchangers. Undersized heat exchangers will result in excessively high barrier fluid temperature, which rapidly decreases mechanical seal reliability.
Barrier Fluid Flow Rate
The barrier fluid flow produced by the mechanical seal pumping ring needs to be sufficient to enable the transfer of the heat absorbed into the barrier fluid—mostly seal frictional heat generation and heat soak from the pump—to the heat exchanger. An insufficient flow rate will result in large differential temperatures between the barrier fluid in and out connections at the mechanical seal and an increase in the overall temperature of the barrier fluid.
Excessively high barrier fluid temperatures result in a number of problems that can affect the performance and reliability of the mechanical seal. Pumps with shaft speeds below 1,800 rpm need a more thorough assessment of the barrier flow rate due to the decreased performance of pumping rings at lower speeds.
The position of the heat exchanger relative to the mechanical seal is important. It should be located as close as possible to and a short distance above the mechanical seal’s centerline (without obscuring access for pump maintenance). Tube or piping connecting the mechanical seal to the heat exchanger should be selected to produce minimal resistance to barrier flow. Large diameter bores, smooth radius bends, short distances and minimal ancillary equipment added into the circulating loop all help lower the resistance to barrier fluid flow.
The accumulator can be mounted a substantial distance from the barrier fluid circulating loop without any impact on the system’s performance. Many installations will mount the heat exchanger and accumulator together on the same stand, provided it does not detrimentally impact the positioning of the heat exchanger relative to the mechanical seal.