Understanding seal chamber pressures part 3

Ron Astall knows pump engineers want a stable liquid film at the seal faces. Venting, cooling and pressure control are ways of achieving this.

Previously, he outlined the role of primary seal flush plans as providing a better environment for our mechanical seals.

To reiterate:

Why have a seal flush?

API 681 / ISO10349 standards define standardised piping plan layouts, which are helpful across all pump types and applications. A selection of common primary piping plans and their expected impact on seal chamber conditions are shown here:

^**depending on seal chamber design.

In the last article, we looked at the simpler plans 01, 11, and 13. These plans provide flow through the seal area, which may limit heat build-up and, with appropriate pump construction, adjust liquid pressure at the seal faces. Pressure management in the seal chamber can be critical if we pump liquids close to their vapour pressure. That is, for liquids that are close to boiling (vaporising). For these situations, providing additional, positive cooling to lower the vapour pressure is a great advantage. Other difficulties that we may face include solids or just plain awkward and unpleasant liquid properties. Following is a selection of additional, more sophisticated piping plans that can be very helpful.

Plan 23 Cooling Loop

This plan involves a recirculation loop through the seal chamber via an external cooler. A pumping ring in the seal is required to positively recirculate flow through the cooling loop. Typically, there will be a throat bush at the inboard end of the seal chamber to limit the mixing the hotter liquid in the pump with the cooled liquid in the seal chamber. This makes for an efficient arrangement because it reduces the ongoing heat load at the cooler. This simplified diagram does not show the high point vents (Fig 5), which ensures the cooling loop is fully primed with liquid. Vapor locking must be avoided. Ideally, piping runs will be sloped appropriately to encourage a thermosyphon flow when the pump is not operating. Inverted loops or high points must be avoided.

Plan 23 will not increase seal chamber pressure, but it is excellent at reducing the temperature. This improves the pressure margin above the liquid vapour pressure and reduces heat transfer along the shaft to the bearings. It is the “go-to” plan for water above 80° C, particularly in Boiler Feed Pumps. Above 80° C, water loses its lubricity and may even flash (vaporise) at the seal faces. For similar reasons, this plan is also standard in chemical and hydrocarbon services when there is a risk of flashing the pumped liquid at higher temperatures. The continuous recirculation aspect allows much smaller coolers than a once-through system. As mentioned earlier, the most critical design and installation tip is to achieve complete venting.

Plan 31 Cyclone Separator

Similar in principle to Plan 11, Plan 31 provides a flush from the discharge to the mechanical seal, with the additional feature of a cyclone separator. The cyclone will separate out limited small solids, with the “clean” leg going to the seal and the “dirty” leg returning to the pump suction. See Fig 6.

As with Plan 11, this provides a flow through the seal, preventing heat build-up with the advantage of a degree of solids removal. A cyclone works best where the specific gravity of the solids is at least twice that of the pumped liquid, such as with sand or pipe slag. Unlike a filter, the cyclone does not require cleaning. However, Plan 31 is not suitable for large solids or slurry applications. The differential pressure between the high-pressure connection and the seal chamber & pump suction must be sufficient to drive the cyclonic action – typically 150 kPa as a minimum.

Plan 32 External Clean Injection

Install Plan 32, and the Mechanical Seal will be your best friend. Properly engineered with a compatible clean liquid, the seal will have ideal operating conditions. A throat bush at the inboard end of the seal chamber will often be installed to boost the chamber pressure and prevent dilution of the clean liquid (see Fig 7). So why not use Plan 32 all the time?

First, a suitable clean liquid must be economically available. Second, the liquid must be compatible with the pumped liquid and the process. Common concerns are whether the product’s dilution is acceptable and whether there will be removal and disposal costs associated with the introduced flush. If specialised, the flush liquid can be an expensive consumable.

One elegant approach I have seen is using a Plan 32 flush to metered inject a process additive via the pump seal chamber. One disaster I have seen is using Light Cycle Oil for a Plan 32 flush. The Pump was operating at 365° C. The Light Cycle Oil simply vaporized as it entered the seal chamber, making the seal’s life worse, not better. Nevertheless, where viable and convenient, Plan 32 is a great way to manage a Mechanical Seal’s environment.

What’s the importance of a seal flush?

The standards define standardised piping plan (flush) layouts, which are helpful across all pump types and applications. A seal flush plan is often indicated to provide a better environment for the mechanical seal. The aim is to provide a stable liquid film at the seal faces.

An appropriate seal flush plan can manage seal chamber venting, cooling flow, and pressure control. Our discussion has focused on the common “primary” seal piping plans, which directly affect the seal chamber conditions.

The next step is to determine the type of mechanical seal. The “primary” seal flush plans above are probably all we need for single seals in nonhazardous service. Dual seals will usually come into play when sealing particularly nasty, dangerous, flammable or toxic liquids. We then need to consider the myriad of specialised “secondary” seal piping plans.

Secondary Seal plans manage the conditions inside the seals, which may be incredibly complex. A fancy dual seal management system may cost more than the pump itself.

Secondary seal systems are a topic for another day.