Proper safety precautions for selecting, installing and maintaining seal systems
When it comes to operating mechanical seal systems in pumps, safety is a prime concern. The potential hazards associated with operating and maintaining pumps' seal systems improperly include fire, explosion, contamination and adverse health and envi-ronmental effects. The results can be catastrophic.
By understanding seal systems and operating them correctly, users can minimize safety hazards. Keeping things running smoothly will extend the life of the seal system and pumping equipment, saving the end user significant costs.
The Seal System
The term “seal system” refers to all aspects of the seal and its use in the pump, including:
- The type of seal, seal materials and any adaptive hardware
- The seal's configuration, including whether mating rings are rotating or stationary; the seal face technology; the relative orientation of any dual seals to each other, such as face-to-back, back-to-back or face-to-face; and the seal's leakage containment design
- The seal's accessories, including interconnecting pipe work between any dual seals and the seal's support system
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| Sealing system components |
The seal system's role in a pump is to minimize leakage of the pumped liquid from the area in the pump where the drive shaft enters or exits the casing. Depending on the type of pumped liquid, leaking fluid can be extremely dangerous, so seal systems help keep pumping equipment operating safely. Users should follow plant and local emergency response system rules when selecting a suitable seal system. Fluid can also leak from flange seals, gaskets or piping connections mounted to the casing directly or indirectly, so users should monitor those potential safety hazards as well.
Seal Safety Issues
Mechanical seals include parts that wear out over time. Seals generally operate with a low rate of acceptable controlled leakage, but if the seal is approaching the end of its life or fails prematurely, that leakage rate can climb to unacceptable levels.
Causes of premature seal failure include:
- Incorrect seal system assembly
- Incorrect pump installation
- Incorrect pump operation
- Failure of another pump component
- Changes in process conditions
In the past, a large percentage of component seals failed prematurely because they were incorrectly installed. The growing use of cartridge seals has eliminated many of these failures.
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| Proper fastener tightening is the first step in successful installation. |
The risks due to excessive seal leakage depend on the pumped liquid's properties, the leakage rate and the surrounding envi-ronment. If the liquid is above its auto-ignition temperature, it can catch fire spontaneously or explode. A fire or explosion can also occur if the liquid or its vapors encounter an ignition source.
Hazards can also occur if seal faces are not lubricated properly. Most pump mechanical seals are wet designs, meaning that the radial face must be lubricated and cooled by a liquid, which, depending on the configureation, can be the pumped liquid or a separate buffer/barrier liquid. Without lubrication, the seal's rubbing surface can overheat leading to a fire or an explosion.
Selecting a Safe Seal
When selecting a seal system, pump users should ensure that the system will work with the pumped liquid at the pump's operating conditions. Conditions to consider include the operating pressure, temperature, maximum shaft speed and the external supply of any cooling, heating, quench, flush, barrier or buffer fluid. If any aspect of the pumping operation changes, review the seal system to ensure that it is still compatible.
Single and dual seal systems offer different types of safety mechanisms to mitigate leakage. Pump operators can use containment bushings to manage leakage in single seal systems. Dual seal configurations can include outer containment seals or pressurized barrier liquid to control leaking, which are smart options for particularly hazardous process fluids.
Some seals and support systems also feature alarms to warn users if leakage-rate levels are unacceptable. Users should know what steps to take if an alarm occurs and act promptly.
Seal System Installation
Installing the seal system properly is crucial for safe operation. Users should read all instruction manuals and keep these to-dos in mind:
- If the system includes a heavy cartridge seal that requires a crane, the seal should have a tapped hole in the gland plate for an eyebolt. The type, number and location of eyebolts that are needed vary depending on whether the equipment has a vertical or horizontal shaft, so use the correct kind.
- Tighten bolts, nuts and fasteners evenly to the specified torque levels, and fasten screws securely. Over-tightening gland bolts can create gland plate distortion, resulting in seal leakage.
- Align the shaft and gland plate correctly. If possible, check the drive shaft after assembly to make sure it can move freely.
- Verify that each port on the seal's cartridge is connected to the appropriate service. Refer to the specified API piping plan for guidance. If ports on the seal are not used, plug them with metal plugs instead of plastic plugs, which have lower temperature limits and can melt.
- Train your staff on how to respond to alarms.
- Ensure that interconnecting pipe work specifications minimize flow resistance and follow venting advice.
- Consult the seal's vendor with questions.
Operating the Equipment
Once the seal systems are installed, users should review the entire pump assembly to ensure that everything works correctly.
- Check the pump at the coupling to make sure the driver or motor is aligned properly.
- Make sure that the motor is rotating in the correct direction.
- Ensure that all alarms and transmitters are operating.
- Make sure that all valve positions are set correctly.
- If the support system includes a heat exchanger and the coolant supply includes isolating valves, install a pressure relief valve between the isolating valves to avoid the risk of explosion.
Proper pressures throughout the system are particularly important for safe operation. If the system includes a barrier system, the system's pressure should not fall below the seal chamber's pressure. The seal supplier may recommend a buffer/barrier fluid pressure based on the original operating conditions, but end users must
