Sealing Sense
Rethinking Flush Water for Mechanical Packing
Improving flush system reliability through pressure, flow rate and system design.
Drew Dixey & Ron Frisard | Fluid Sealing Association members, A.W. Chesterton Company
| IMAGE 1: Flush objectives (Images courtesy of Drew Dixey, assisted by ChatGPT)
| IMAGE 2: Factors that determine flush performance
“What flush flow rate should I use?” is a common question asked when designing a flush system for a packed stuffing box. While it seems straightforward, it is often the wrong place to start. There is no universal flush flow rate for flushing compression packing. In fact, not every packed stuffing box equipped with a flush port requires a flush. The amount of flush required depends on the application’s objective, equipment design and operating conditions. Before selecting a flow rate, engineers should first ask: “What is the flush intended to accomplish?”
Once that objective is understood, pressure, flow and system design become much easier to evaluate.
Why Are We Flushing?
A flush should solve a specific problem, not simply provide visible water flow. A common assumption in packed pump applications is that a flush is always required. Instead, the first question should be: “Why is the flush needed?”
Depending on the application, a flush may serve several different purposes. In slurry service, it creates a pressure barrier that prevents abrasive solids from entering the packing. Flush may prevent the formation of deposits in applications where media crystallization is a concern. High-temperature hydrocarbon applications often use flush to prevent coking, while other services rely on flush to remove heat generated by friction or the process itself.
Before selecting pressure or flow, determine the primary purpose of the flush
- Excluding solids
- Preventing crystallization
- Cooling the stuffing box
- Preventing coking
- Protecting the packing
- Improving reliability
Reliability is not improved just because water is flowing. Excessive flush can waste water, dilute product, increase treatment costs and even reduce packing life. In some applications, modern packing systems may require only minimal flushing—or none at all.
Why There Is No Universal Flush Flow Rate
Once the objective has been established, the next question becomes how much flush is required. Unfortunately, no single flow rate works for every application. Flow rate is only one part of the system; pressure, stuffing box geometry, packing material, operating conditions and equipment design all influence how much flush is needed to accomplish the intended purpose.
Stuffing Box Pressure Comes First
Flush water cannot enter the stuffing box unless its pressure exceeds stuffing box pressure. A common starting point is maintaining flush pressure about 1 bar (14.5 pounds per square inch [psi]) above stuffing box pressure.
When direct measurement is unavailable, a rough estimate for many end-suction pumps is:
This is only an approximation since pump designs vary significantly. Using pump discharge pressure as the basis for flush pressure often creates excessive pressure and flow, leading to higher leakage, water consumption and packing wear.
Once the pressure relationship has been established, the resulting flow will depend on factors such as shaft speed, process temperature, lantern ring position, packing arrangement, equipment clearances and the permeability of the packing itself.
| IMAGE 3: Misaligned lantern ring
Pressure Creates the Barrier; Flow Verifies Performance
One of the most common mistakes is setting a target flow rate and increasing pressure until that flow is reached. This reverses the correct approach.
The proper sequence is:
- Determine the purpose of the flush.
- Establish the required flush pressure relative to stuffing box pressure.
- Observe the resulting flow.
- Verify that solids exclusion, cooling, lubrication or deposit prevention objectives are being met.
The optimum flush rate is therefore the minimum amount of clean fluid required to perform the intended function under the worst operating conditions. Because pump designs, processes, packing materials and operating conditions vary, there is no universal flush flow rate.
Is the Flush Actually Working?
Installing a flush system does not mean it is accomplishing its intended purpose. Once the system is operating, there are three questions that can help determine whether the flush is performing effectively.
- Is the flush water clean? Since flush is intended to keep abrasive particles out of the stuffing box, the flush water itself should not introduce new contaminants. Suspended solids in the flush supply can cause sleeve wear and reduce packing life. Clean and filtered flush water is recommended.
- Is the flush consistent? Flush needs to remain stable throughout the operational life of the equipment. Variations in plant water pressure or manual on/off valves that can be closed or partially closed can reduce effectiveness. If flush pressure falls below stuffing box pressure, process fluid can also backflow into the flush supply line, potentially contaminating or completely blocking the line. An appropriately selected inline check valve can help prevent reverse flow and protect the flush supply. A consistent flush is often more important than a higher flow rate.
- Is the flush reaching the packing? Utilizing the correct pressure and clean water will provide no benefit at all if the flush never reaches the intended location. A misaligned lantern ring or blocked flush port may allow the water to bypass the sealing interface entirely. Verifying lantern ring location during installation can prevent many common flush-related problems.
| IMAGE 4: Flush performance checklist
Flush System Troubleshooting
When a packed pump is not performing as expected, increasing flush pressure or flow is rarely the best solution. Instead, begin by identifying the symptoms and considering the possible causes. Many flush-related issues can be corrected without increasing flow. Understanding the original purpose of the flush and verifying that clean water reaches the packing at the proper pressure will often resolve problems more effectively than opening the valve further.
We invite your suggestions for article topics as well as questions on sealing issues so we can better respond to the needs of the industry. Please direct your suggestions and questions to sealingsensequestions@fluidsealing.com.
Drew Dixey is a design engineer with A.W. Chesterton Company and currently serves as chair of the Fluid Sealing Association’s mechanical packing division. He may be reached at drew.dixey@chesterton.com.
Ron Frisard is the global product line director – mechanical packing and gasketing with A.W. Chesterton Company, with more than 35 years of experience in fluid sealing technology. He serves as programs chair for the Fluid Sealing Association. He may be reached at ron.frisard@chesterton.com.
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