High Temperatures Create Dangerous Pressure Increases in Dual-Seal Systems

Climate has an often ignored impact on sealing systems. Damage from harsh weather conditions, such as intermittent storms or humidity, can be effectively managed. But daily exposure to the outdoors, especially where extreme temperatures are the norm, can wreak havoc on seals.

Preventing Leaks with Pressure

Dual seals, Arrangement 3, as specified in American Petroleum Institute (API) 682, form a system where two seals prevent process fluid from escaping into the environment. The cavity between the two seals is maintained at a pressure that is higher than the process pressure in the seal chamber. The difference in pressure ensures that any leaks will be of the barrier fluid into the process or atmosphere and that the process will always be contained.

This arrangement is also used where the process fluid has insufficient lubricating properties or is loaded with solids that would damage seal faces. In this case, the barrier, not the process, fluid is sealed. A schematic of the arrangement is shown in Figure 1.

Figure 1. The schematic above shows a dual-seal system according to Plan 53A. (Graphics courtesy of the author)

A pumping ring typically circulates barrier fluid between the two seals. Natural convection also occurs when the heated fluid exits the top of the seal and cooled fluid re-enters the bottom of the seal. The tank is filled to a given level above the seal connections, and a volume of gas above the liquid pressurizes the system. The tank is then isolated from the pressurized gas source.

This design is classified as Piping Plan 53 and has several versions. This article will focus on Plan 53A, but also applies to Plan 53B.

Hotter Climates, More Pressure

In a pressurized dual-seal system, the barrier pressure should always be above the process pressure. While this pressure differential is essential to the success of the seal, rarely is the seal cavity pressure measured with a gauge. Instead, end users estimate the pressure depending on the design and construction of the pump, operating conditions and piping plans that modify the seal chamber pressure. The maximum pressure may easily be estimated at 7 bar (0.7 MPa, 101.5 psig) for what might be a normal seal cavity gauge pressure of 5 bar (0.5 MPa, 72.5 psi). The barrier fluid pressure would then be set at least one atmosphere higher than the maximum seal cavity pressure, or 8 bar (0.8 MPa, 116 psig). This pressure level would have to be maintained at all times, including at the minimum ambient temperature.

Ambient temperature changes cause pressure variations in the barrier-fluid tank used to circulate the barrier fluid for a dual seal. The system must be set so that the desired pressure is maintained at the lowest ambient temperature. The result is that at the maximum ambient temperature, the system will end up at a higher pressure.

In one example, the minimum ambient temperature is 5 degrees Celsius, and the maximum ambient temperature is 45 C. Equation 1 calculates the change in pressure.

Equation 1

Where:
P_x = Absolute pressure at the higher temperature
P_y = Absolute pressure at the lower temperature
T_max = High absolute temperature
T_min = Low absolute temperature

In this example, the absolute pressure at the lower temperature is 8 bar (0.8 MPa, 116 psig).

Liquid level varies because of the natural leakage that occurs at the seals over time. Because of the relatively small volume of gas in the tank, the pressure will vary with the change in gas volume. The tank's operating liquid volume depends on the allowable period between refills. A normal liquid level (NLL) in the barrier fluid tank (see Figure 2) is usually about two-thirds of the visible level. A high liquid level (HLL) is close to the top of the sight glass, and a low liquid level (LLL) would be close to the bottom. The operating range is between the normal and low liquid level. A high liquid level would indicate leakage of process fluid into the tank.

Figure 2. The fluid level fluctuates between low (LLL), normal (NLL)and high (HLL).

In a typical example, when using a 12-liter (L) tank, the minimum liquid level would be 6 L, and the normal liquid level would be 8.5 L. Equation 2 calculates the pressure increase for the normal level compared with the low level.

Equation 2

Where:
P_N = Absolute pressure at normal level
P_L = Absolute pressure at low level
V_max = Maximum gas volume
V_min = Minimum gas volume

According to this simplified equation, the absolute pressure at the normal level will be 15.7 bar (1.57 MPa, 228 psi).

Other Sources of Heat

The seal also generates heat during operation.The heat is transferred to the tank and dissipated into the environment. An equilibrium above the ambient temperature will result.

For this example, a typical increase in temperature is 25 C above ambient. At the maximum ambient temperature conditions, this increase results in a barrier fluid temperature of 70 C. The change in pressure can be calculated using Equation 1, where 15.7 bar now serves as the pressure at the low temperature, or P_y. The resulting pressure at the high ambient temperature is 16.9 bar (1.69 MPa, 246 psi).

Solar radiation is a particularly important factor in hot climates, such as the Middle East or North Africa, where outdoor temperatures are some of the most extreme in the world. If the pump is outdoors in full sunlight, solar radiation could easily increase the temperature to 90 C. The daily temperature changes will have a significant impact on pressure ranges.

To account for the temperature increases from solar radiation, the temperature in this example now rises from 45 to 90 C. Using Equation 1, the resulting absolute pressure from the solar radiation is 17.9 bar (1.79 MPa, 260 psi).

In this example, a process pressure of just 5 bar could result in a barrier pressure of 16.9 bar—about 12 bar above the process pressure. The goal is to maintain only 1 bar above process pressure.

Figure 3. Prolonged exposure to solar radiation increases tank temperature and pressure.

One other factor to contend with is that many dual seals are double-balanced on the inboard or primary seal. However, the balance ratio is often significantly higher from inner pressure than outer pressure. Frictional heat from the imbalance further increases the temperature in the tank, which leads to even higher pressure. This self-perpetuating cycle can easily result in reduced seal life, or even failure.

A final consideration is that a temperature rise is not the only condition to guard against. A temperature drop below the freezing point of the barrier fluid will interrupt circulation and most certainly lead to seal failure.

Solving the Pressure Problem

For outdoor installations, the following tips can protect seal life against the dangers of rising pressure from high temperatures:

• Install the tank somewhere with constant shade.
• Use the largest tank possible. Larger tanks reduce pressure variation for the same variation in barrier fluid volume.
• Check balance ratios and choose a seal designed for high barrier fluid pressure on the primary seal.
• Use a pressure regulator to maintain constant pressure. The regulator must allow venting to reduce pressure when the tank temperatures are too high. A regulator will not be the best option if process vapors mix with the barrier fluid.

The analysis applies to Plan 53A (gas over liquid) and Plan 53B (gas separated from the liquid by an elastomeric bladder). An alternative approach is Plan 53C. A differential piston effect pressurizes the barrier fluid (1:1.2 or 1:1.5) so that the barrier fluid pressure will change as the seal chamber pressure changes. It maintains constant differential pressure across the primary seal. However, the plan only allows a small volume for barrier fluid storage, and the pumped fluid must be clean.