Looking to convert Circulating Water Vertical Pumps and Circulating Water Makeup Vertical Pumps from fluid injected seals to waterless mechanical seals to save on water usage. I was wondering whether the technology is sound and whether or not anyone has had experience with the waterless seals. Any information will be appreciated. Thanks,
Are those applications with high percentage of solids?
If so there are a few seals out there that started out in the Mining industry that don't use a Plan 32 (external flush to cool, clean, or keep solids from the seal faces). The seal faces will usually still be wetted by the process fluid, some may be a version of a dry gas seal (I wouldn't bet on that though).
Many times they will stilll use water as a quench, Plan 62 (a fluid injected on the atmospheric side of the seal to add or remove heat from the faces, or to control oxidation of process fluids) to cool the faces. The quench won't enter the process. The quench fluid is usually contained by a close tolerance bushing or a lip seal of some sort. You will have to make provision for the quench fluid to go somewhere that won't be a saftey hazard or a housekeeping issue.
Some are used as pressurized or non-pressurized dual seals too, depending upon the concentration of the solids. An easy explanation of the non-pressurized version is that the seal uses a second mechanical seal to contain a quench. The pressurized version of the seal will also use a second mechanical seal but the fluid between the two will be at a higher pressure than the process fluid in the seal chamber of the pump. In this case the pressurized barrier fluid (water or a thin oil) will leak into the process at atmosphere at a very low rate- you'll hardly notice. The advantage is that it provides cool, clean lubricaton for the seal faces and that you have positive containment of the process fluid as long as there is high pressure fluid between the two seals.
Another thing to think about is the seal chamber itself. If your pumps use small bore seal chambers like those used with packing, you may want to think about upgrading to large or taper bore seal chambers. Larger seal chambers promote better cooling and less turbulent circulation of the process fluid around the seal. Less turbulent flow means less abrasion of the hardware and seal faces.
Important things to look for are robust seal faces and hardware. Depending upon the application multiple springs may be acceptable, and others may require a large single springs. Again depending upon the application, springs in the process fluid may be acceptable, others may requre the springs to remain outside to keep from clogging.
As mentioned above your trusted seal vendor will be a big help.
Dan, Longeron has provided a good explanation I wish to add a little more to this. The technology is there and if you provide more information on why you have water injection (API Plan32) then we may be able to be more specific with a reply. Essentially you should consider Silicon Vs Silicon Carbide balanced cartrdge seals with the correct piping plan. For vertical pumps you will need API Plan 13. If you dead end the seal you may experience dry running so ensure you vent the seal cavity. If this is a water application then there is no need for a quench also bear in mind that lip seals are not reliable when used with mechanical seals. Just check what your stuffing box pressure is operating at so you can decide on the piping plan. I think dual pressurised seals will be too expensive for this application but is an option if you experience problems.
Venting the seal cavity in a vertical pump application is the most critical step to remember in this case. The plan 13 is useful in applications where the seal chamber pressure is at or near discharge pressure. My personal preference if it can be done is to use a plan 14 as the seal cavity will then self vent. This cannot always be accomplished in most vertical applications; again, it depends on the seal chamber pressure. Using a dead ended seal chamber in a vertical application is ill-advised for the vapor/venting concerns noted. Dual seals are an option, but will add cost and complexity to the support system. As previously stated, an accurate evaluation of the process fluid and operating details with the seal vendor is recommended.
