Heat Exchangers -- Keep Out of Hot Water

Some simple steps can ease heat exchanger commissioning.

By Dirk Willard, Contributing Editor

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I knew we were in trouble. The client, despite protests from me and operators with a few hundred years of experience, decided to run the heat exchanger without flushing the cooling tower. The water was sludge. We wasted a day cleaning out the plates.

Commissioning heat exchangers poses all sorts of dangers. Let's review the basics so you don't collect similar anecdotes.

A walk-through is essential and is the starting point for a successful commissioning.

A walk-through is essential and is the starting point for a successful commissioning. First, collect the data sheet on the exchanger and the drawings. Next, check the punch list to confirm pressure testing and cleaning have been completed. Verify that at least a visual inspection, especially of the tubesheet, tubes or plates, was done at the shop.

During the walk-through: 1) Ensure all inlets and outlets are fitted for temperature and pressure gauges; 2) Check foundation bolts -- they should be loose at one end, preferably in a pipe sleeve for easy adjustment; 3) Allow space for equipment removal -- four feet or so is necessary to take out a floating head or plates (You need a plan for safe maintenance); 4) Look at vent cocks and relief devices -- they should be sized correctly and vented to a safe spot; 5) Verify the exchanger can be drained easily and hazardous waste is confined by a dike around the unit; 6) Make certain flow through new pipe to the unit goes through a filter or, at a minimum, a strainer; 7) Confirm all unused nozzles are blocked and flanges and other joints have correct and new gaskets; and 8) Check that relief valves are installed where they should be for thermal expansion of liquids -- if liquids flow through the exchanger.

After the walk-through, consider a pneumatic leak test. Soap all joints and tighten as required. The ASME code only allows testing up to maximum allowable working pressure or 50 psig -- whichever is lower. The leak rate for N2 is 52 times that for water, so this is a good test before initial startup; helium is only about 1.12 times N2 and hydrogen is 2.2 (assuming laminar flow). Leak testing will help drive out moisture. However, the test won't find a tubesheet leaking into a shell -- that's the shop's job.

Now it's time to go over the job safety analysis for the startup procedure. Ensure all safety equipment is in place and all instrument loops are functioning, and review contingency plans for safety and environmental problems.

The type of heat exchanger to some degree affects the initiation procedure. Exchangers with fixed tubesheets offer the greatest challenge. With fixed tubesheets: 1) Start hot flow first with a condensing gas (e.g., steam) in the shell; or 2) Begin both flows simultaneously when using liquids in both the shell and tubing. Otherwise, for other fixed tubesheet configurations (U-tubes, packed floating heads, packed floating tubesheets and internal floating heads) start cold flow first, then hot.

Shutdowns are the opposite of startups. For example, with a U-tube heat exchanger, gradually close off the hot flow, then shut the cold flow. There is one difference though: drain all steam condensate -- slowly! It's not a good idea to leave an exchanger full and unused even if only for a few weeks; corrosion and freezing can cause damage, especially to delicate tubing. So, either drain an idle exchanger and purge it with dry gas or periodically circulate fluids.

Make an allowance for venting, especially if an exchanger is full of inert gas or where condensing steam could pull a vacuum on tubing or a shell not rated for full vacuum. And don't forget to vent to a safe location. Adjust all flows slowly and once the exchanger reaches desired operating temperature keep it there! Heat exchangers containing long thin tubes don't handle thermal shock well -- so avoid cycling them!

Your job doesn't end once the exchanger is operating. Don't forget to close the vents! Tighten bolts loosened for thermal expansion. Identify any potential maintenance problems, such as water hammer, poorly supported pipe, and relief lines that don't allow for condensation drainage. Pinch points have been one of the leading dangers in working with heat exchangers lately and have led to at least one fatal accident. Workers have been crushed aligning a head or tubesheet. Review heat exchanger installation and make it easier and safer.

After commissioning the exchanger take the time to document the process. Practice makes perfect.


DIRK WILLARD is a Chemical Processing contributing editor. You can e-mail him at dwillard@putman.net

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