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by Thomas Webster on Tue, Jul 13, 2021 @ 15:07 PM
Dual mechanical seals are often used in Alberta refineries when pumping applications involve hazardous fluids to provide an extra level of safety. To maintain proper lubrication and heat removal to dual mechanical seals, a seal flush system is required. Unpressurized configurations require the use of a buffer fluid, whereas pressurized configurations use barrier fluids.
API Plan 52 is the most commonly used seal flush system for unpressurized dual mechanical seals and circulates buffer fluid between inboard and outboard seals. If preventative measures are not taken, API plan 52 failures can occur and result in downtime. Let’s take a look at how the plan works for dual mechanical seals, common API Plan 52 failure modes, and how to prevent such failures.
API Plan 52: An Overview
API Plan 52 is often used for refinery applications involving light hydrocarbons or fluids with high vapour pressure, but may also be used for lower vapour pressure process fluids. This plan delivers and circulates an unpressurized buffer fluid between inboard and outboard seals from an external reservoir (seal pot). The reservoir typically contains an internal cooling coil or external water or air cooler to remove heat. An internal pumping ring, located between the inner and outer seals of the dual seal configuration, provides positive circulation through the flush system and into the seal chamber while the pump is running. API plan 52 provides lubrication, flushing, and cooling to the outboard seal and cooling to the inboard seal.
The advantages of API Plan 52 include:
- Lack of process contamination
- No direct leakage of process fluid to the atmosphere
- Not required to maintain pressure system, as seen in Plan 53A
Common API Plan 52 Failure Modes
When a dual mechanical seal leaks or fails with an API Plan 52 seal flush system, the most common causes of failure are heat checking, coking, corrosion, and blistering on seal faces. Below we have detailed common causes of seal failure for dual seals utilizing Plan 52, the associated symptoms, potential root causes, and solutions.
|
Failure Mode |
Symptom |
Potential Root Causes and Solutions |
|
Heat Checking |
Heat checking is characterized by radial cracks originating from the center of metallic, tungsten carbide, or ceramic seal faces. Heat checking is typically caused by insufficient lubrication, inadequate cooling, or a combination of both. |
Potential solution: If heat checking is severe, the mechanical seal may need to be replaced or upgraded to a more resilient material. If heat checking is minor, take measures to ensure the seal flush system is providing the proper lubrication and cooling to seal faces. |
|
Coking |
Coking appears as a black build-up or abrasive sludge on the atmospheric side of the seal face and can compromise the mechanical seal performance due to the impeded buffer fluid flow. |
Potential solution: When coking is discovered, use steam to remove debris and sludge from the atmospheric side of the seal. For a more permanent solution, API Plan 62 can be utilized, as it is designed to prevent coking on seal faces. |
|
Corrosion |
Corrosion occurs as the result of metal atoms being oxidized by a fluid, resulting in loss of material on the surface of the seal face. Corrosion and pitting are often the results of a chemical attack on the seal face material. |
Potential solution: If corrosion develops, assess chemical makeup of process fluid to determine the correct material for seal components and faces. |
|
Blistering |
Blistering can occur as the result of a thermal attack and is characterized by raised circular sections on carbon seal faces. Misalignment of faces due to blistering can cause seal leakage. |
Potential solution: If blistering is minor, take measures to ensure proper cooling and circulation of the seal face. If blistering is severe, consider upgrading the seal to non-porous material. |
Proactive Measures to Reduce API Plan 52 Failures
While addressing heat checking, coking, corrosion, and blistering in Alberta refineries may often mean replacing parts depending on the severity, there are a number of proactive measures that can be taken to reduce the likelihood of failure of dual seals with an API Plan 52 flush system. Here is a quick checklist of preventative measures to improve cooling and lubrication efficacy and efficiency:
- Assess process conditions to determine if the appropriate buffer fluid is being used to prevent heat checking.
- Check and replace buffer fluid on a regular basis to ensure contaminated fluid is not causing coking.
- Check pumping ring functionality to prevent insufficient circulation of buffer fluid.
- For highly viscous fluids, such as heavy crude oil, ensure that a buffer fluid is selected that will not readily mix with process fluid and reduce cooling properties.
- As process conditions change, determine whether or not the buffer fluid and process fluid are still compatible.
- Assess cooling coils on a regular basis and descale if necessary.
- Implement additional instrumentation and evaluate functionality of existing instrumentation, such as a pressure switch in the seal pot that allows for early detection of high pressure. Note that the buffer fluid should be maintained at a pressure less than the seal chamber and less than 40 psi. An increase in seal pot pressure can be a sign of inboard seal leakage.
- Avoid running pumps when the seal pot level is low, as indicated by a level gauge.
- Verify the seal pot vents remain open at all times.
Still have questions on how to improve API Plan 52 cooling and lubrication efficiency? Field Advisors at Edmonton Valve & Fitting can perform an onsite or remote inspection of your process and seal flush system to determine the cause of inefficiencies. We can make recommendations on upgrades, component materials, tubing diameters, and instrumentation to help deter API Plan 52 failures.