Seal pot pressure, the barrier pressure of an API Plan 53A, 53B or 53C dual seal, is the maximum seal chamber pressure plus a margin of at least 1.4 bar (20 psi), and that margin has to hold at the coldest ambient temperature the system sees. The calculation is the same for all three plans; what changes is the setting on each system that delivers it.

Seal pot pressure = maximum seal chamber pressure + margin (API 682 minimum: 1.4 bar / 20 psi)

If the barrier falls below seal chamber pressure, the inner seal is reverse-pressurized and process fluid can cross into the barrier loop.

Step 1: Find the Maximum Seal Chamber Pressure

Estimate the maximum seal chamber pressure from the pump’s design, its operating conditions and the piping plan feeding the chamber, and take the highest value that estimate gives, not the normal operating reading. Seal chamber pressure is rarely measured with a gauge, so the barrier is set from this estimated maximum.

Step 2: Add the Barrier Pressure Margin

Add at least 1.4 bar (20 psi) to the maximum seal chamber pressure; that is the minimum barrier margin API 682 recommends. In practice, barrier pressure normally runs 1.4 to 4.1 bar (20 to 60 psi) above seal chamber pressure.

The margin has to hold at the lowest ambient temperature. In a 53A reservoir or a 53B bladder, the nitrogen that pressurizes the barrier fluid loses pressure as it cools, so make the setting for the coldest condition.

Step 3: Apply the Pressure for Your Plan

Only a Plan 53A is set directly to the target pressure from Steps 1 and 2. On a 53B and a 53C, the target is the minimum the system must deliver, and the settings that produce it are calculated back from it.

Plan 53A

On a Plan 53A, set the nitrogen regulator on the reservoir to the target pressure; the regulator holds it constant from the plant gas header. The set point cannot exceed the plant nitrogen header pressure, so a target above it needs nitrogen bottles.

API 682 advises against using Plan 53A above 150 psig (about 10 bar); above that pressure, the duty normally moves to a Plan 53B.

Plan 53B

A Plan 53B is set with three pressures, all worked back from the target:

  • Refill alarm: the minimum barrier pressure, equal to the target from Steps 1 and 2.
  • Pre-charge: the nitrogen pressure in the bladder before any barrier fluid goes in, set just below the refill alarm.
  • Full set point: the pressure the loop reaches when barrier fluid is pumped in; it decays back toward the refill alarm as the seal consumes fluid.

API 682 pairs a Plan 53B pre-charge of 99 % of the minimum barrier pressure with a minimum retained liquid of 1 % of the accumulator volume at that minimum pressure. The pre-charge also changes with temperature. The 53B nameplate lists it against ambient temperature, so note the temperature where the bladder is charged and read the pre-charge from that chart.

The full set point follows from the combined gas law:

P1 × V1 / T1 = P2 × V2 / T2

Subscript 1 is the bladder at pre-charge and subscript 2 is the condition being checked. P is absolute pressure (gauge plus atmospheric), T is absolute temperature (kelvin or Rankine), and V is the gas volume: the accumulator volume less the liquid in it.

The liquid pumped in is the working volume. A Plan 53B is set so that the working volume lasts at least 28 days between refills, with API 682’s safety factor of two on the leakage rate.

Most 53B systems use a fixed refill alarm and a fixed full set point, calculated so the margin and the refill interval both hold across the site’s ambient temperature range. Systems with a floating full set point come with a chart of the set point against temperature instead.

Plan 53C

On a Plan 53C, barrier pressure is the reference pressure multiplied by the piston’s amplification ratio, so the ratio is chosen together with the point on the pump that supplies the reference.

Barrier pressure = reference pressure × amplification ratio

The amplification ratio is the area of the piston face on the reference side divided by the area on the barrier side, which the piston rod reduces. Plan 53C amplification ratios are typically 1.05 to 1.30.

The margin over the reference shrinks as the reference pressure falls:

Margin = reference pressure × (amplification ratio − 1)

Check it at the bottom of the reference point’s pressure range. If the margin there falls below 1.4 bar (20 psi), take the reference from a higher pressure: the final discharge, or an intermediate stage on a multistage pump. The alternative is a piston accumulator with a helper spring that adds a minimum pressure offset.

Step 4: Set the Low-Pressure Alarm

Set the low barrier pressure alarm between the maximum seal chamber pressure and the barrier set point, so it trips while the barrier is still above the seal chamber.

On a Plan 53B, the low-pressure alarm is the refill alarm, set at the minimum barrier pressure from Steps 1 and 2. An optional low-low alarm, used to shut the pump down, goes between the maximum seal chamber pressure and the refill alarm.

A fixed 53B refill alarm is calculated at the minimum barrier liquid volume and the maximum ambient temperature. Calculated at a cooler temperature, the alarm lets the accumulator run dry on a hot day before it trips. On cold nights a fixed alarm trips early, and a refill then can over-pressurize the system once the day warms up, so a pressure alarm with a temperature bias is recommended for 53B.

On a Plan 53A, a level switch or level gauge signals the refill, and the reservoir also carries a low-pressure alarm. Whatever offset that alarm is given, check that it still sits above the maximum seal chamber pressure.

Step 5: Check Against the Pressure Ratings

Check the highest pressure the barrier will reach, not the set point, against the maximum allowable working pressure (MAWP) of the mechanical seal, the seal pot or accumulator system, and the pump. The highest pressure depends on the plan:

  • Plan 53A: the regulator setting, provided the regulator can relieve; a gas space that cannot vent rises with ambient temperature.
  • Plan 53B: the full set point at the hottest ambient temperature.
  • Plan 53C: the highest reference pressure multiplied by the amplification ratio.

On the double seal itself, the inner seal has to be designed to take the barrier pressure, so check its balance ratio against the peak barrier pressure as well as the process pressure.

If the peak exceeds a rating on a 53B with fixed set points, a larger accumulator narrows the gap between the full set point and the refill alarm and lowers the peak.