Just like you may want a clean desk and a comfy chair, mechanical seals appreciate good working environments too. The best way to ensure that your mechanical seal is performs at its best, is to select the seal properly and have the appropriate flush plan to support it.
What is a Flush Plan?
A flush plan is a strategic equipment arrangement that circulates fluid to or from the seal chamber in order to maintain proper lubrication, pressure, temperature, and solids management. The goal is to create an ideal work environment for the seal in order to avoid wear and failures, thus prolonging its useful life.
There are many types of flush plans that are selected for certain applications, fluids, and seal configurations. Keep in mind that seals and flush plans are engineered products that require calculated design in their selection, so be sure to talk to an engineer about them!
How Do They Work?
There are some flush plans that use the pump construction geometry to function, but we will be focusing on external flush plans in this post. Flush plans typically depend on redirecting fluid to or from the seal chamber by using tubing equipment and fluid treatment devices. The fluid can either be from the process itself or from an external source, depending upon what is required for the process. Typically, the simpler the process is, the simpler the flush plan can be. Hazardous or dangerous fluids will require more aggressive flush plans to protect the materials and workers from exposure to the product.
Simple plans can use the suction or discharge pressure of the pump to move fluid across the seal faces – these plans will simply have piping with orifices leading from a port to the seal chamber. Connecting to the discharge port will allow for the high pressure discharge fluid to travel to the seal chamber and re-enter the process. Connecting to the lower pressure suction port will cause fluid to move from the seal chamber to the suction port to re-enter the process. Either way, you are providing lubrication and cooling while flushing solids away from the seal so that it can maintain appropriate contact and keep your process inside the pump.
Sometimes the process fluid needs to be treated before it can re-enter the seal chamber. Plans in this category pipe the fluid from the pump ports and use external devices, such as heat exchangers or cyclone separators, to cool the fluid down or remove solids before it passes over the seal.
In the tough cases, the process fluid may not be suitable for supporting the seal and an external fluid source will be required. Many times, cool plant water will do the trick. This will dilute the process product as seal water is introduced into the system, but many times that is acceptable.
When you have specialized fluids that should not leak to the atmosphere or will not permit dilution with seal water, a double mechanical seal will be required and the flush plan will act as a miniature loop of barrier or buffer fluid. Barrier fluid is pressurized and keeps the product inside the pump. Buffer fluid is unpressurized so it collects leaking product and creates a diluted solution. These plans will be more elaborate with reservoirs and monitors, but they offer the benefit of excellent seal environment control and great protection from exposing the product to the atmosphere.
Why Should You Care?
You spent time and money on the equipment and product, didn’t you? Incorporating the correct flush plans will help protect the equipment that you have while preventing future issues. Taking care of your seals with a flush plan will increase their life expectancy and minimize unplanned downtime and expensive repairs. Also, a well cared for mechanical seal will help reduce leaks and wasted product, keeping you from mopping dollars off of the floor. Finally, in tough applications, they are absolutely essential for safety reasons.
If you are interested in setting up a flush plan for your mechanical seals, contact us! Our experienced engineers are ready to help you design the perfect plan for your application.
When discussing slurry pump seal support plans it seems that the terms “Flush” and “Quench” are often confused and or misused. With the concept being slightly different for mechanical seal boxes vs packed seal boxes I will separate these, discussing each in turn.
Mechanical Seal
The basic mechanical seal flush plan is very simple. It calls for a clear/clean liquid, usually water, to be introduced into the space between the actual seal and an impeller side exit restriction. The flush liquid is introduced at a pressure above that of the pumpage, thus assuring a positive outward flow/flush and a clean operating environment for the mechanical seal. This is illustrated below.
To quote API 682 3rd edition:
The definition of a flush is a “fluid which is introduced into the seal chamber on the process fluid side in close proximity to the seal faces and typically used for cooling and lubricating the seal faces.
When flushing is required, Toyo recommends the use of an API plan 32 seal piping arrangement as it is better suited in services containing solids or contaminants which could damage the seal faces if recirculated in the flush media.
The Plan 32 typically uses a clear clean fluid supplied from an external source delivered to the primary seal faces on the process fluid side of the seal. With the use of a close-clearance throat bushing, the stuffing box can be back pressured to an elevated pressure ensuring the flushing fluid will not flash across the seal faces.
A quench plan, as the name indicates, is designed to quench or cool the seal. It is normally used if short periods of dry running are expected. As illustrated bellow, the fluid is introduced into the area between the back of the seal faces and the close fit exit on the drive side of the pump.
API 682 3rd Edition defines a quench as a “neutral fluid, usually water or steam, is introduced on the atmospheric side of the seal to retard formation of solids that may interfere with movement, or for other purposes.”
Some quench seal plans replace the close fit exit restriction with a secondary seal and a top exiting port that can be plumbed to capture the spent quench fluid and route it away from the rotating assembly. The principal is however just the same, we are trying to cool the seal not in any way flush the seal.
Cautionary note: A common mistake is to over-pressure the box and damage the expensive mechanical seal.
Packed Box Seals
The primary goal of all flush water plans is to preclude the pumpage from contaminating the seals. As such flush water plans for packed boxes are very similar to that of mechanical seal boxes. However, as illustrated below there are some obvious mechanical differences. The most striking difference is the addition of a seal (packing) located between the injection port and the exit restriction. This minimizes the volume of flush liquid consumed.
From an operational standpoint, the packed box differs as it requires some leakage to assure lubrication and prevent heat build-up. The mechanical seal box should have no leakage.
When working with a packed box, whether it be a flush, the rule of thumb for setting water pressure is the same. The pump shut off pressure, plus 10% or plus 20 psi, which ever is greater. There is however a difference in how flow rates are set.
With a standard flush plan, flow is typically adjusted by compressing the packing until a few drops are observed leaking out of the seal on the drive side. On a quench plan, flow rate is set by adjusting an inlet valve while at the same time using a valve on the exhaust side to maintain the correct seal box pressure. If the exit water from the seal box is too hot, flow rate is increased until exit water is cool while still maintaining the proper seal box pressure.
I hope this short blog helps clear up some of the confusion with seal flush plans. Before signing off for today, let me confirm the information provided today is general info only. Always refer to the pump manuals for specific details. If still in doubt, our applications team here at Toyo is always willing to support you in any way they can.