API Plan 53C is an API 682 piping plan for pressurized dual seals in which a piston accumulator, driven by the pump’s own pressure through a reference line from the seal chamber, holds the barrier fluid at a fixed ratio above seal chamber pressure. Because the pump drives the accumulator, the barrier pressure rises and falls with the pump, while a pumping device in the seal circulates the barrier fluid through a cooler.
Plan 53C serves Arrangement 3 dual pressurized seals, as Plans 53A, 53B and 54 do. It is sometimes called a pressure boosting or pressure intensifying system, after what the piston does.
How the Piston Accumulator Pressurizes the Barrier
The barrier fluid comes out above seal chamber pressure because pumped fluid from the reference line pushes on the full face of a free piston, while the barrier fluid pushes back on the other face, where the piston rod takes up part of the area.
The amplification ratio is the area of the process-side face divided by the area of the barrier-side face.
As normal seal leakage consumes barrier fluid, the piston rises and the rod extends further out of the top of the cylinder, so the rod’s extension shows how much barrier fluid is left.
Refilling means pumping barrier fluid into the accumulator up to the full level the accumulator’s maker specifies.
How Plan 53C Pressures Are Set
Two choices set the barrier pressure in a Plan 53C: the accumulator’s amplification ratio and the point on the pump that the reference line is connected to.
Barrier pressure = reference pressure × amplification ratio
Barrier Pressure Above the Seal Chamber
With the seal chamber as its reference, a Plan 53C typically holds the barrier fluid about 10% above seal chamber pressure, a ratio of 1.1, and most accumulator ratios fall between 1.05 and 1.30.
API 682 also recommends a barrier pressure at least 1.4 bar (20 psi) above the seal chamber. Because a Plan 53C margin is a percentage of the reference pressure, it has to clear 1.4 bar at the lowest pressure the reference point sees in operation, not only at the duty point.
Choosing the Reference Point
On a high-pressure pump, the reference line is connected to the seal chamber or to a port in the seal gland plate, and the ratio alone clears the 1.4 bar minimum. On a low-pressure pump, seal chamber pressure multiplied by the ratio falls short, so the reference is taken from a higher-pressure point: the pump’s final discharge, or the discharge of an intermediate stage on a multistage pump.
The other option on a low-pressure pump is an accumulator with a helper spring, which adds a fixed minimum offset on top of the ratio.
How Barrier Pressure Follows the Pump
When the pump’s pressure changes, the barrier pressure changes with it in proportion to the amplification ratio, because the pumped fluid acts on the piston all the time.
How Barrier Fluid Circulates and Is Cooled
A pumping device inside the seal, usually a pumping ring, drives the barrier fluid around a closed loop through a cooler and back to the seal.
The cooler is either an air- or water-cooled heat exchanger in the loop, as on a Plan 53B, or an internal cooling coil. An internal cooling coil typically limits a Plan 53C’s cooling capacity to 4 kW.
When Plan 53C Is Used
Plan 53C is chosen for high-pressure pumps and for pumps whose operating pressure varies over a wide range, where the pressure differential across the inner seal has to stay within a narrow band. It also suits sites with no nitrogen supply, since the pump itself powers the accumulator and no gas sits on the barrier fluid to dissolve into it at high pressure.
No threshold specific to Plan 53C sets a pressure above which it should be chosen. The 10 bar (145 psi) figure sometimes given as one comes from the second edition of API 682, which advised it as a maximum for Plan 53A to limit gas absorption into the barrier fluid.
Plan 53C is the most complex of the three Plan 53 systems: its piston and piston seals are moving parts that need periodic maintenance.
When Plan 53C Is Not Used
A standard Plan 53C is ruled out wherever the pumped fluid is not clean, because the pumped fluid itself fills the reference line and drives the piston. Four kinds of pumped fluid are avoided:
- Fluids that solidify at ambient temperature: almost nothing flows in the reference line, so a product that solidifies as it cools to ambient plugs the line and the barrier pressure is lost.
- Fluids carrying suspended solids: solids that migrate into the accumulator wear the piston seal and the cylinder wall until barrier fluid leaks past the piston.
- Hot fluids: product that enters the accumulator at commissioning can be hotter than the piston seal material tolerates.
- Corrosive fluids: the product wets the accumulator walls and piston seals, so their materials must resist it.
A Plan 53C built for contaminated or corrosive products is a modified system, in which nitrogen fed through a pressure tracking valve drives the piston and the reference line is isolated from the product and filled with a clean fluid. On abrasive and contaminated services, users have traditionally chosen Plan 53A or 53B instead.