Double booster pump air seals, adapted from compressor air seal technology, are more common in the shaft seal industry. These seals provide zero discharge of the pumped liquid to the atmosphere, provide less frictional resistance on the pump shaft and work with a simpler support system. These benefits provide a lower overall solution lifecycle cost.
These seals work by introducing an external source of pressurized gas between the inner and outer sealing surfaces. The particular topography of the sealing surface puts additional pressure on the barrier gas, causing the sealing surface to separate, causing the sealing surface to float in the gas film. Friction losses are low as the sealing surfaces no longer touch. The barrier gas passes through the membrane at a low flow rate, consuming the barrier gas in the form of leaks, most of which leak to the atmosphere through the outer seal surfaces. The residue seeps into the seal chamber and is eventually carried away by the process stream.
All double hermetic seals require a pressurized fluid (liquid or gas) between the inner and outer surfaces of the mechanical seal assembly. A support system is required to deliver this fluid to the seal. In contrast, in a liquid lubricated pressure double seal, barrier fluid circulates from the reservoir through the mechanical seal, where it lubricates the seal surfaces, absorbs heat, and returns to the reservoir where it needs to dissipate the absorbed heat. These fluid pressure dual seal support systems are complex. Thermal loads increase with process pressure and temperature and can cause reliability problems if not properly calculated and set.
The compressed air double seal support system takes up little space, requires no cooling water, and requires little maintenance. In addition, when a reliable source of shielding gas is available, its reliability is independent of process pressure and temperature.
Due to the growing adoption of dual pressure pump air seals in the market, the American Petroleum Institute (API) added Program 74 as part of the publication of the second edition of API 682.
74 A program support system is typically a set of panel-mounted gauges and valves that purge the barrier gas, regulate downstream pressure, and measure pressure and gas flow to mechanical seals. Following the path of the barrier gas through the Plan 74 panel, the first element is the check valve. This allows the barrier gas supply to be isolated from the seal for filter element replacement or pump maintenance. The barrier gas then passes through a 2 to 3 micrometer (µm) coalescing filter that traps liquids and particulates that can damage the topographical features of the seal surface, creating a gas film on the surface of the seal surface. This is followed by a pressure regulator and a manometer for setting the pressure of the barrier gas supply to the mechanical seal.
Dual pressure pump gas seals require the barrier gas supply pressure to meet or exceed a minimum differential pressure above the maximum pressure in the seal chamber. This minimum pressure drop varies by seal manufacturer and type, but is typically around 30 pounds per square inch (psi). The pressure switch is used to detect any problems with the barrier gas supply pressure and sound an alarm if the pressure drops below the minimum value.
The operation of the seal is controlled by the barrier gas flow using a flow meter. Deviations from seal gas flow rates reported by mechanical seal manufacturers indicate reduced sealing performance. Reduced barrier gas flow may be due to pump rotation or fluid migration to the seal face (from contaminated barrier gas or process fluid).
Often, after such events, damage to the sealing surfaces occurs, and then the barrier gas flow increases. Pressure surges in the pump or partial loss of barrier gas pressure can also damage the sealing surface. High flow alarms can be used to determine when intervention is needed to correct high gas flow. The setpoint for a high flow alarm is typically in the range of 10 to 100 times the normal barrier gas flow, usually not determined by the mechanical seal manufacturer, but depends on how much gas leakage the pump can tolerate.
Traditionally variable gauge flowmeters have been used and it is not uncommon for low and high range flowmeters to be connected in series. A high flow switch can then be installed on the high range flow meter to give a high flow alarm. Variable area flowmeters can only be calibrated for certain gases at certain temperatures and pressures. When operating under other conditions, such as temperature fluctuations between summer and winter, the displayed flow rate cannot be considered an accurate value, but is close to the actual value.
With the release of API 682 4th edition, flow and pressure measurements have moved from analog to digital with local readings. Digital flowmeters can be used as variable area flowmeters, which convert float position into digital signals, or mass flowmeters, which automatically convert mass flow to volume flow. The distinguishing feature of mass flow transmitters is that they provide outputs that compensate for pressure and temperature to provide true flow under standard atmospheric conditions. The disadvantage is that these devices are more expensive than variable area flowmeters.
The problem with using a flow transmitter is to find a transmitter capable of measuring barrier gas flow during normal operation and at high flow alarm points. Flow sensors have maximum and minimum values that can be accurately read. Between zero flow and the minimum value, the output flow may not be accurate. The problem is that as the maximum flow rate for a particular flow transducer model increases, the minimum flow rate also increases.
One solution is to use two transmitters (one low frequency and one high frequency), but this is an expensive option. The second method is to use a flow sensor for the normal operating flow range and use a high flow switch with a high range analog flow meter. The last component the barrier gas passes through is the check valve before the barrier gas leaves the panel and connects to the mechanical seal. This is necessary to prevent the backflow of pumped liquid into the panel and damage to the instrument in the event of abnormal process disturbances.
The check valve must have a low opening pressure. If the selection is wrong, or if the air seal of the dual pressure pump has low barrier gas flow, it can be seen that the barrier gas flow pulsation is caused by the opening and reseating of the check valve.
Generally, plant nitrogen is used as a barrier gas because it is readily available, inert and does not cause any adverse chemical reactions in the pumped liquid. Inert gases that are not available, such as argon, can also be used. In cases where the required shielding gas pressure is greater than the plant nitrogen pressure, a pressure booster can increase the pressure and store the high pressure gas in a receiver connected to the Plan 74 panel inlet. Bottled nitrogen bottles are generally not recommended as they require constant replacing empty cylinders with full ones. If the quality of the seal deteriorates, the bottle can be quickly emptied, causing the pump to stop to prevent further damage and failure of the mechanical seal.
Unlike liquid barrier systems, Plan 74 support systems do not require close proximity to mechanical seals. The only caveat here is the elongated section of the small diameter tube. A pressure drop between the Plan 74 panel and the seal can occur in the pipe during periods of high flow (seal degradation), which reduces the barrier pressure available to the seal. Increasing the size of the pipe can solve this problem. As a rule, Plan 74 panels are mounted on a stand at a convenient height for controlling valves and reading instrument readings. The bracket can be mounted on the pump base plate or next to the pump without interfering with pump inspection and maintenance. Avoid tripping hazards on pipes/pipes connecting Plan 74 panels with mechanical seals.
For inter-bearing pumps with two mechanical seals, one at each end of the pump, it is not recommended to use one panel and separate barrier gas outlet to each mechanical seal. The recommended solution is to use a separate Plan 74 panel for each seal, or a Plan 74 panel with two outputs, each with its own set of flowmeters and flow switches. In areas with cold winters it may be necessary to overwinter the Plan 74 panels. This is done primarily to protect the panel’s electrical equipment, usually by encasing the panel in the cabinet and adding heating elements.
An interesting phenomenon is that the barrier gas flow rate increases with decreasing barrier gas supply temperature. This usually goes unnoticed, but can become noticeable in places with cold winters or large temperature differences between summer and winter. In some cases, it may be necessary to adjust the high flow alarm set point to prevent false alarms. Panel air ducts and connecting pipes/pipes must be purged before placing Plan 74 panels into service. This is most easily achieved by adding a vent valve at or near the mechanical seal connection. If a bleed valve is not available, the system can be purged by disconnecting the tube/tube from the mechanical seal and then reconnecting it after purging.
After connecting the Plan 74 panels to the seals and checking all connections for leaks, the pressure regulator can now be adjusted to the set pressure in the application. The panel must supply pressurized barrier gas to the mechanical seal before filling the pump with process fluid. The Plan 74 seals and panels are ready to start when the pump commissioning and venting procedures have been completed.
The filter element must be inspected after a month of operation or every six months if no contamination is found. The filter replacement interval will depend on the purity of the gas supplied, but should not exceed three years.
Barrier gas rates should be checked and recorded during routine inspections. If the barrier air flow pulsation caused by the check valve opening and closing is large enough to trigger a high flow alarm, these alarm values may need to be increased to avoid false alarms.
An important step in decommissioning is that isolation and depressurization of the shielding gas should be the last step. First, isolate and depressurize the pump casing. Once the pump is in a safe condition, the shielding gas supply pressure can be turned off and the gas pressure removed from the piping connecting the Plan 74 panel to the mechanical seal. Drain all fluid from the system before starting any maintenance work.
Dual pressure pump air seals combined with Plan 74 support systems provide operators with a zero-emission shaft seal solution, lower capital investment (compared to seals with liquid barrier systems), reduced life cycle cost, small support system footprint and minimum service requirements.
When installed and operated in accordance with best practice, this containment solution can provide long-term reliability and increase the availability of rotating equipment.
We welcome your suggestions on article topics and sealing issues so that we can better respond to the needs of the industry. Please send your suggestions and questions to sealsensequestions@fluidsealing.com.
Mark Savage is a product group manager at John Crane. Savage holds a Bachelor of Science in Engineering from the University of Sydney, Australia. For more information visit johncrane.com.
Post time: Sep-08-2022