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Fluid Handling: Effectively Purge Lines

Jan. 19, 2022
Follow some proven pointers for getting rid of loose materials

An important commissioning activity prior to startup is line cleaning to remove debris. Loose material, which can include scale, sand, rust, dirt, plastic or fibers, can wind up blocking downstream equipment, plugging tower packings and valves, fouling heat exchangers, etc. The surest way to minimize loose debris in equipment is via careful cleaning and inspection before closing the equipment. The most-effective way to deal with pipes or ducts is to use a pig to physically push out the debris.

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However pigging frequently isn’t practical for large pipe networks or when time is short. An alternative often used is to purge with a flush fluid or air. While a purge can’t remove every item, it can eliminate items that likely would come loose in normal operation.

The purge fluid imposes a drag force on material left in the pipe. If the drag force is high enough, the material moves to a collection point, e.g., a strainer or a trash container by an open pipe end.

For turbulent flow, you can determine drag force, Fd, using Newton’s law:
Fd = kUρν2 (1)

where k is a constant defined by the particle shape (geometry), U is a coefficient related to the flow regime (Reynold’s number), ρ is density of the purge fluid, and v is the velocity of the purge fluid compared to the velocity of the material to be moved.

If you don’t know what’s in the pipe, you can’t specify k and, so, can’t directly calculate force. Nevertheless, the drag equation gives us a good idea of how to evaluate the velocity needed for a fluid flush or blow.

As noted, a purge can’t remove every possible item from a pipe. For example, moving a dropped wrench would require a lot of force. However, you can compare the amount of force in a purge to drag forces in normal operation. This ratio, C, is:
C = Fd cleaning/Fd operating = kUρcleaning fluid νcleaning fluid 2/
kUρoperating fluid νoperating fluid 2 (2)

With the assumption that k is constant regardless of the purge fluid and velocities for purging will be high enough so U is roughly a constant, this reduces to:

C = ρcleaning fluid νcleaning fluid 2operating fluid νoperating fluid 2 (3)

If the purge can achieve a C much greater than one, it will move materials that normal operation doesn’t. Any items still left shouldn’t cause problems because normal operation won’t move them.

Take care in defining the normal operation that requires protection. Is it the usual operating rate, a higher operating rate that’s expected, the rate for an abnormal shutdown condition, or some other rate? Once the normal operating rate is set, you can determine the flush rate.

Based on experience, water flushes and air blows should target a C value of at least 1.2 but preferably higher. It’s good to aim for a C value of at least 1.5. You also must provide enough time to allow the purge to clean out the longest run. Of course, even high C values won’t guarantee a clean pipe. Large and high-density objects always are difficult to move. Past experience can be invaluable.

For water washes, estimating velocities is relatively simple: the system pump rate should give a total flow, and standard hydraulic calculations provide a velocity estimate. In air blows, figuring the rate can be more complex. Air blows often are done by pressurizing the system with air and then suddenly depressurizing the system. This uses the system volume to hold inventory and reduces the compressor size required to deliver air. In such cases, air flow rate is the relief rate, which varies with system pressure.

Finally, ensure the receptacle for the flushed materials can deal with their impact. Purged items may arrive with significant kinetic energy. The trash collector must handle this effectively and safely. In particular, an air blow large enough to remove relatively large scale will impart a very high velocity on small items. If the air blow dumps to an open container, confirm that safety steps are taken to handle possible dust and high-velocity particles entering the container. Steps may include, e.g., dust suppression and prevention of static electricity build-up.

Correctly and safely done, a purge is an important step that improves plant reliability.

ANDREW SLOLEY is a Chemical Processing Contributing Editor. You can email him at [email protected]

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