Editor's Note: This article is the fourth in a six-part series on seals. For other articles in this series, click here.
This article discusses the operating conditions and design limits; applications for single seals, tandem seals, tandem seals with intermediate labyrinth and double seals. It also explains tandem seals versus double seals.
To select a dry gas seal configuration, the operating conditions and design conditions must be determined. Both the operating conditions and the design conditions will influence the type of seal used for an application. To ensure a dependable dry gas system, seal conditions that must be identified are normal sealing pressure, maximum sealing pressure, operating temperature, seal design temperature and process gas.
Normal Sealing Pressure
The first factor to be identified is the normal sealing pressure. This is the pressure that the seal must contain within the compressor during normal operation. For beam-style compressors, this is typically suction pressure. However, some configurations, such as back-to-back compressors, can have higher–than-suction sealing pressures. For overhung-style compressors, the sealing pressure is somewhere between suction and discharge. The discharge pressure gas decreases in pressure as it moves down the backside of the compressor impeller to the seal location.
Therefore, based on the design of the compressor, the actual sealing pressure must be confirmed with the compressor manufacturer. Knowing the actual sealing pressure will provide accurate leakage information to reference during operation and for designing the seal monitoring system.
Maximum Sealing Pressure
The next pressure information needed is the maximum sealing pressure, typically the design pressure for the seal. Since the discharge pressure is the maximum pressure identified for the compressor, this could be used for the maximum sealing pressure. Seals will typically never seal the discharge pressure of the compressor, so the pressure rating will be much higher than the maximum operating pressure of the seal. Designing the seal to discharge pressure can also be expensive.
For example, there is usually a price change for seals at about 1,500 psi to 1,700 psi, depending on the manufacturer and then another price change at about 3,000 psi to 3,300 psi. Verifying the maximum sealing pressure required for the application can lower the cost of the seal. Usually, the seal will never operate at higher than the settle-out pressure of the compressor, estimated at the mid-pressure point between suction and discharge pressure. Normally, this is an estimate since the volume of gas upstream and downstream of the compressor to the unit valves will influence the settle-out pressure.
A larger volume of gas from compressor to discharge valve compared to the volume of gas from the compressor to suction valve will result in a higher than midpoint settle-out pressure. If the larger volume of gas is on the compressor to suction-valve side, then the settle-out pressure will be below the midpoint. If the discharge pressure is below the 1,500 psi to 1,700 psi for lower-pressure applications or 3,000 to 3,300 psi for higher-pressure applications, then using discharge pressure as design pressure is not a big concern. If settle-out and discharge pressures straddle the identified pressure, then more time may be required to identify the actual design pressure for the seal.
Settle-out pressure can also effect the decision to use a tandem seal or a double seal. Therefore if the application is low-pressure and/or for a dirty gas, identify the true settle-out pressure. Choosing a double seal for these services can provide a much more reliable seal. For dry gas seal applications, low pressure is below 100 psi.
Operating Temperature and Seal Design Temperature
The next step is to identify the actual operating temperature and the seal design temperature. On a beam compressor, the suction-end seal and discharge-end seal will operate at different temperatures. No compressor original equipment manufacturer (OEM) can provide an exact temperature for the seal operating temperature because typically no temperature probes are at the seal, so the operating temperature is assumed to be somewhere between the process gas temperature and the bearing temperature.
Standard seals, from most manufacturers, can manage temperatures of 300 F to 350 F. If the discharge temperature is below this, then use the discharge temperature as the seal design temperature. If the discharge temperature is above this, then more time may be required to identify the actual design temperature if eliminating unnecessary costs is a concern.
The other point about seal design temperature is seal supply gas temperature. If there is an issue with the dew point of the seal supply gas, then the gas should be heated to manage the dew point. The temperature of the seal gas should be considered when identifying the seal temperature rating. As indicated, seal temperatures are somewhere between process gas and bearing temperature. If the seal experiences discharge temperature, the bearing must be designed to handle this temperature as well because the seal is not far from the bearing.