ACHRNEWS

Steam Trap Troubles? Skip the Psychics

June 26, 2000
Figure 1. Different processes require different traps.
In the first part of this series, we took a broad look at the different types of steam traps a service tech could come across in the field.

This week, let’s delve into the specifics of capacity, failures, sizing and selection, maintenance, repair, and more.

Capacity

Determining the capacity of a trap is relatively easy given the manufacturer’s data.

Capacity is going to depend on how large the orifice size is and the pressure difference across the trap. If we change either, the capacity changes — very simple!

We want to size a trap based on the maximum condensate it will experience and the minimum pressure differential. That will ensure we have a trap that can handle the worst-case situation.

Also, to ensure that we do get a minimum pressure, we need to stack condensate above the trap. For every 2.31 ft of condensate above the trap, we have 1-psi differential. We would like a minimum differential of 0.5 psi. Therefore, the trap needs to be at least 15 in. below the device it is draining.

Features

Picking the right trap depends not only on what you are trying to accomplish, but also on many other factors that are occurring in the system.

Is it a constant load or a variable load? How would you prefer the trap to fail? Is subcooling acceptable? Is there a chance for water hammer? Is it a batch load where air removal occurs often? Is it a process that never shuts down, so air removal is not a major concern? How dirty is the system?

No one trap is the answer for all situations. Luckily, most manufacturers have a selection guide or flow chart to help in selecting the right trap.

You just have to make sure that the trap can adequately meet the features you require. Knowing basically how each trap works (as described in Part One of this series, The News, June 19) will go a long way in selecting the right one.

Figure 2. Don't get trapped by using the master trap.

Failed Traps

What happens when a steam trap fails? As we have noted last week, it can fail one of two ways: open or closed.

If it fails closed, condensate is not drained. It backs up in our device and the transfer of latent heat is halted.

If it fails open, steam will blow through the trap. Again, we will lose out on latent heat transfer. In this case, not all the steam will get the opportunity to give up its latent heat, but some of the steam will still heat the process.

A failure open will be costly in terms of energy wasted. The condensate that you have heated and treated will leave the system through a vented receiver. Now you have to replace the lost condensate with cold city water, heat it, and treat it.

Knowing the price of steam and how much you are replacing, you can quickly calculate its cost.

Another problem caused by a failed open trap is that it can prevent condensate from draining from other traps. How?

Remember, the capacity of a trap is determined by orifice size and pressure differential. If there is no pressure differential across a trap, then its capacity is zero.

How does a failed trap do this to other traps? Imagine 15-psi steam blowing past a failed trap into the condensate return line. Now the return line is pressurized to 15 psi. The pressure on the discharge side of any trap draining into the same line as the failed trap is now much higher than anticipated, so we no longer have a pressure differential across the other traps. Hence, there is no condensate drainage.

Then there is the problem of the steam from the failed trap collapsing in the condensate line and causing water hammer. But that is not an issue we will discuss today.

What a Trap Can’t Do

As stated earlier, a trap cannot pump. Only pumps pump.

Having said that, a trap cannot lift condensate to an overhead wet return. A difference in pressure can lift condensate.

Because we want to be conservative, we use a common, and safe, rule of thumb that says when lifting condensate, we lose 1 psi for every 2 ft we lift the condensate. As the pressure drops, and the hotter the condensate is, the more likely it will flash into steam.

When it collapses, the result is water hammer. So we have to be very careful when lifting condensate. The best way to lift condensate is with a pump.

Sizing and Selection

We size the steam trap based on the maximum condensate load and minimum pressure differential.

Remember, if you have a lift situation, you have to take into account the static pressure due to a lift on the discharge side of the trap. Then you determine what type of trap is required.

Then you add a safety factor based on the type of trap you are going to select. The safety factor will also depend on the warm-up load and how confident you are with your other numbers. The less confident you are, the greater the safety factor you should use.

Now you are ready to select a trap. Based on everything you have determined, you look at the manufacturer’s catalog.

Good news — the manufacturer probably has software that will ask you some questions to determine the capacity you require, as well as questions that will walk you through an electronic selection guide.

Inspection, Maintenance

Dirt and loose scale are usually the biggest problems for steam traps. It is important to remove these contaminants, as they will damage the seat and pin of a trap.

There may be a dirt pocket or strainer upstream of your trap. Clean the pocket or strainer. Inspect the trap.

Some traps are easier to inspect than others. For example, the thermodisc trap is very simple. Remove the top slowly, clean off any dirt, and check for wear of any internal components. Replace necessary parts.

A good way to tell when the disc in a thermodisc trap is worn, is that it will cycle at a rapid rate. Some refer to it as “machine gunning.”

For the other traps, it gets a little more difficult. Without opening the trap, you may be able to tell if the trap has failed open. That is to say, steam is blowing through.

In the old days, service/maintenance techs would check the temperature on both sides of the trap by using wax sticks that melt at different temperatures.

If they had the same temperature on both sides of the trap, there was a possibility that the trap had failed open. If there was a large temperature drop across the trap, that would indicate that the trap had failed closed.

Now, instead of using wax sticks, we use an infrared temperature sensor. All you have to do is point it at the location at which you want to measure the temperature. It’s a great device for getting the temperature at these types of hard-to-reach locations.

However, paint, dirt, and dust can affect the accuracy of such a device, so its main benefit lies in recognizing a temperature difference across the trap, not the actual temperature.

This may be stating it simpler than it actually is. Remember, the thermostatic trap requires subcooling, so expect to see a larger temperature drop.

Other traps discharge at saturation temperature. They will not have as great of a temperature drop. See how knowing the basic operations of a trap are vital for troubleshooting?

There are other easy ways you can also use to check the operation of the trap without opening it.

If some forethought was put in the installation and the installer added a test valve, you’ve got it made. Or better yet, the manufacturer provided a test connection.

Close the return shut-off valve and open the test valve. Now you can observe if the trap is leaking live steam. You may also get flash steam, which is OK. The difference between the two is that flash steam is slow moving and billowy as it leaves the test port. Live steam will leave the test port straight and with some velocity.

But before doing this test, remember, we’re dealing with steam here. Be very careful.

Another popular way to test a trap is through sound. With a listening device and a little experience, you can hear if a trap is working properly. These devices can be as crude as a screwdriver held up to the ear, or as sophisticated as an electronic device that can filter out unwanted sounds.

The inverted bucket trap is probably the easiest to hear in operation. Just listen for the clinking sound of the bucket as it cycles up and down. If it rattles instead of cycling, you have lost your prime.

We’ve already discussed the thermodisc trap and its sound.

The other trap types may take a little more experience. There are even very sophisticated devices that will report to your computer if a trap has failed. As these are still expensive and not widely used, we will only mention that they exist.

All these methods will also tell you if the trap has failed closed. From our earlier discussion, realize that a trap may not be draining condensate because there is no pressure differential.

In other words, a trap that has failed open somewhere else may be placing back pressure on the discharge side of other traps that are draining to the same condensate return lines. Take that in account when you think a trap is defective because it is not draining condensate. There may be nothing wrong with that particular trap, but another one.

For the mechanical and thermostatic traps, check the seat and pin for erosion, grooving, and wire drawing.

On the thermostatic and float and thermostatic (F&T) traps, look at the thermal element and the float. Any damage to these items will mean that they have to be replaced.

The inverted bucket and the orifice plate traps have a small orifice. On the inverted bucket, make sure the vent hole is not blocked. On the orifice plate, it’s obviously the orifice.

Trap Repair

Repair of most traps is easy. Simply replace the parts that are damaged.

Traps are generally designed so that they do not have to be removed from the system in order to repair them. Often, all that is required is to remove a few bolts, replace the guts of the trap, bolt it back together, and you are back in business.

You can usually order the guts and assembly cover as one part so all you have to do is remove the old cover with attached guts and replace it with the new cover with attached guts.

Lastly, let me mention one very valuable program, the trap survey. This is probably the most important tool of all.

At a minimum, list where all your traps are, and what type and make of traps they are.

Gathering this information will be well worth your time and effort. There are several people who are willing to do it for you at a price. I personally think everyone needs to do it at least once. You will learn a lot just doing it.

The Master Trap: A Common Mistake

A common mistake in trap applications is to drain multiple devices into a common trap.

Here is the problem. These devices may not have the same load or pressure drop through them. That means one device will discharge condensate at a higher pressure than another device, causing the condensate to back up in other devices.

The lesson here is that each device needs its own trap. However, we can put multiple traps in parallel draining a single device. And if we want to get fancy, we can have a low-pressure trap designed to handle the warm-up load. When the lockout pressure is reached, it locks shut. Then a high-pressure trap handles the operating load.

Fancy, huh? Just don’t get too fancy for your application. Think it through before you try something out of the norm. What may work in one application will not work in another.

Summary

We have covered a lot in these two articles.

We looked at how different types of traps operate. Based on this knowledge, we saw how one trap may be better than another for a certain type of application.

Next, we discussed how we go about selecting the right trap.

Finally, we finished with some basic maintenance and repair instructions.

The theme has remained the same throughout the two articles: Think it through and use common sense. A little knowledge of the working operations of a trap can go a long way in operating a happy and healthy steam system.

(By the way, my wife has tried the psychic hotline. All she got out of it was me.)

Gerhardt gives numerous seminars on service and maintenance for hydronic and steam systems. Presently he is an instructor at the ITT Little Red Schoolhouse in Morton Grove, IL. Gerhardt can be reached 847-966-3700 or mgerhardt@fluids.ittind.com (e-mail).