Circulation Systems for Single and Multiple Seal Arrangements (Part One)


Written by:
Gordon Buck and Ralph Gabriel, John Crane Inc.

Questions are sometimes asked about the maximum flush rate. Although increasing the flush rate beyond the recommended value may produce further improvements, by definition this effect is rapidly diminishing beyond that point.  At very high flush rates and close clearances, erosion can occur.  As an example, when sealing water at 250-psig using a balanced 2-in seal at 3600-rpm, the minimum flush rate might be computed as 0.4-gpm based on an allowable temperature rise of 15-deg F. The rule of thumb yields 2-gpm for a 2-in seal. Therefore, the recommended flush rate would be 2-gpm.

On the other hand, when sealing propane under the same conditions, the minimum flush rate is computed as 2.5-gpm based on an allowable temperature rise of 5-deg F. Thus, for propane the recommended flush rate would be 2.5-gpm.

Pumping Rings

Pumping rings are used in closed loop sealing systems such as Plans 23, 52, and 53A-C to produce flow through coolers and reservoirs. There are two basic pumping ring designs: radial flow and axial flow. Either design can be effective. Just as the performance of a centrifugal pump is a function of the impeller and volute, the performance of the pumping ring depends on the design of the seal chamber. In particular, the design, size and placement of the inlet and outlet ports are crucial to the performance of the pumping ring.

The first rule for pumping rings is to make the diameter of the inlet and outlet ports as large as possible. This includes the pipe tap and the final drill-through. There is no particular reason to make the outlet port smaller than the inlet port unless there is not enough space for both to be large. For radial flow pumping rings, a tangentially directed outlet port is absolutely essential. This requirement applies to all variations of radial flow pumping rings, including those with vanes, drilled holes, slots, paddle wheels, knurled surfaces, etc.

According to simple theory, there is no reason to expect any flow through a radially directed outlet port. In actual practice, a small flow rate, usually about 25 percent of the amount expected from a tangential outlet, is produced by a radial outlet providing that the outlet port is large.

For axial flow pumping rings, the inlet and outlet ports must not be directly over the vanes of the pumping ring. There should be an inlet and outlet region at the ends of the pumping ring to assure even distribution of the liquid. Although a tangential outlet is not essential for axial flow pumping rings, significant performance improvements are realized when the outlet is tangential. In effect, an axial flow pumping ring with a tangential outlet becomes two pumping rings in series.

Unlike outlet ports, inlet ports can and should be radially directed. Just as is the case for centrifugal pumps, a tangential inlet would cause pre-rotation of the liquid, which would adversely affect performance.

In general, the pressure and flow from a pumping ring increases with diameter and shaft speed. Pressure and flow decrease with increased radial clearance.

The circulation rate in a seal system is a function of the fluid properties and system piping as well as the pumping ring. Small piping, numerous directional changes, and viscous liquids result in low flow rates. The procedure for estimating the circulation rate is to first construct a piping system (resistance) curve and then superimpose the pumping ring performance curve. The intersection of these curves defines the circulation rate.

When both the pumping ring and the system are properly designed, circulation rates of about ½-gpm to 1-1/2-gpm per inch of seal size are easily attainable.

Thermosyphon Systems

A thermosyphon is a closed loop system in which fluid flow is produced by gravity through the effects of temperature on density. This natural circulation results from the differential head that exists between the cold and hot sections of the system. The cold fluid has the greater density and displaces the hot fluid. The saying "warm air rises" is better described as "cold air sinks."

 

Thermosyphons can provide cooling for liquid sealing systems; however, great care must be taken because thermosyphon flow rates are small and easily stopped by bubbles from vaporization or dissolved gases. A single bubble that is about the same diameter as the piping can stop flow; this is called vapor-locking. To prevent vapor locking and maximize flow, large diameter piping, connections, and drill-throughs should be used. The cooler or reservoir should be 2-ft to 5-ft above the seal chamber. If thermosyphoning is not a concern a cooler or reservoir height of 1-ft to 2-ft can be used as this will reduce the system resistance slightly. Liquid should flow "in the bottom and out the top" of the seal chamber. The system must be periodically, or continuously, vented. To assist in the thermosyphon effect, the return or hot piping leg should be insulated so that no cooling occurs in this line.

Because of the quirky and sensitive nature of thermosyphons, most specifications require a positive circulation using some type of pumping ring. Even so, the effects of thermosyphoning should always be considered when designing seal circulation systems. That is, the system should always be designed to promote thermosyphoning.

Quenches For High Temperature

A quench, as defined by API 682, is "a neutral fluid, usually water or steam, introduced on the atmospheric side of the seal to retard formation of solids that may interfere with seal movement." Nitrogen is another quench medium.

In high temperature services, a steam quench may provide several benefits:

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