By Tim Daly
Thermodynamics play a huge role in the design and construction of all piping systems. It comes into play with the temperature of the fluids or gases being carried inside of the pipes, and also with the temperatures of the environment that the pipes will be located. We will begin our discussion with steam, and then move on to environmental issues later on.
When we talk about boiler plant settings or steam systems in general, all discussions should begin—and end—with thermodynamics. How the material used for piping and components reacts to the extreme temperatures, and the potential for extreme temperature changes, makes all the difference when the material is selected.
Much of this is ironed out by engineers when a system is designed; however, as equipment ages and newer technologies become available, changes are inevitable. This is where pipefitters and steamfitters must be diligent and pay attention when ordering replacement parts. With boilers and steam, you must know that the components you’re ordering are rated high enough with regards to pressure and temperature. Keeping that in mind, you must also realize what temperature and pressure the system you’re working on will reach. Superheated steam will reach temperatures that can be far higher than the temperature of saturated steam.
A few years ago I was working in a power plant. We had three identical water tube boilers, and each had four solenoid valves that operated automatically to admit atomizing steam to the oil burner tips. One such valve had failed on one of the boilers. A maintenance mechanic, the plant’s instrument control technician, and the maintenance foreman hatched a plan to replace each of the automatically controlled solenoid valves with a manually operated ball valve. They started by replacing the four valves on the boiler where the one solenoid valve had failed.
It should be noted here that any changes to a boiler’s start up and operating procedures should be approved by a licensed boiler inspector, and the plant’s “standard operating procedures” should be updated to include any new steps in the procedure. Also, all personnel should be appraised of and thoroughly trained on the new procedures.
The next time the boiler was started using the new valves, it was noted by the operator that the handle on the ball valve had become so soft and pliable that it was “like butter.” Upon further investigation it was discovered that the valves they had selected were not rated for steam use at all. Luckily, no one was hurt. A good steam valve will have a Steam Working Pressure (SWP) stamp on the side, and manufacturers should provide all necessary details with the valve, or you can check manufacturer’s websites.
The next consideration with regards to steam systems and thermodynamics is expansion and contraction. When metal heats up, it expands, and when it cools, it contracts.
When steam systems are designed, these concepts are thought out carefully and provided for. Expansion joints, flexible joints, and expansion loops are carefully engineered to meet the expansion and contraction of the entire system taking into mind the temperatures and pressures in the various systems, as well as total volume in any particular area. If an expansion joint is sized too small, it will not handle the job and will inevitably fail; or worse yet, another part of the system will rupture instead. Where expansion joints and loops are installed, anchor points will also be installed. Anchor points are provided to direct where the expansion and contraction will take place. If the engineer requires the expansion to occur east of the expansion joint instead of west, he will put an anchor point on the west side of the joint, and a series of rollers on the east side for the expanding or contracting pipe to travel on.
The pipefitters and welders who install these systems must be able to read and understand the blueprints and diagrams provided at the start of the job. It is ultimately their responsibility to ensure that the system is laid out with the right components in the right spaces. A large project may have multiple types and sizes of expansion joints and steam traps. In this instance, when we talk about sizes, we are taking into consideration the amount of expansion that the expansion joint will provide. All expansion joints are not the same, and must not be substituted for one another. This same principal also applies to steam traps. There are many different types of traps, and each type has a specific purpose. Installing an inverted bucket trap, where a float and thermostatic trap should be, may be detrimental to the overall performance of the system. Also, “bushing” down the pipes on a trap station to fit the first trap a fitter comes across is unacceptable. In regards to expansion loops, a north/south run designed to be eight feet needs to be eight feet. “Seven feet’s good enough,” is definitely not good enough.
Pipefitters and welders called upon to make repairs to steam systems must also be aware of the expansion and contraction. For instance, let’s say that a fitter and an apprentice were dispatched to replace a flanged valve. They arrive at the scene of the bad valve, and see it leaking profusely. They know that they need to get a good shut-down of the system, so they find two valves on either side to shut. At this point they have a couple of choices. They can get anchor points welded on either side of the valve to prevent the pipes from traveling when they remove the valve, or they can arrange to shut the system down now, and then wait a few hours or more for any system contraction to take place so the pipes won’t travel on them before they actually break into the system.
All of the situations that we have discussed to this point are even more prevalent when they take place inside of the boiler plant, because the closer you are to the boiler, the higher the pressures and temperatures will be. This is even more prevalent within the boiler itself.
Operators of boilers must always be aware of what is going on inside of a boiler. If boiler tubes are subjected to the heat in the furnace box without steam or water present in the tubes, they will become too hot and ultimately fail. When a boiler operator has a low water condition in his boiler, he must immediately shut the boiler down, and carefully arrest the situation. If he panics and adds cold water to the hot boiler too fast, the shock will result in blown boiler tubes or worse. Similarly, he must also ensure that his water level does not become too high. If the water level gets too high, it becomes subject to what is known as carryover. This is when the steam exiting the boiler picks up water from the boiler and carries it into the steam system. The result here is water hammer.
Here again we get back to steam traps, and we start to get into proper condensate draining before start up, and proper condensate removal during normal operating conditions. Condensate systems must be designed to work properly in conjunction with the steam system, and also the layout of the plant or campus that will be employing the system.
Some plants have the ability to have condensate that can use gravity to flow back to the boiler plant, while others will require pumping. Some installations will use both, and this is where designers need to be on top of their game. If you are dumping high pressure condensate directly into a wet system that is being pumped from a receiver you risk dangerous water hammer.
Operators of steam and condensate systems need to fully understand what is happening when they open a steam valve. They need to know that all excess condensate has been evacuated from the system before introducing steam to it, and that there are adequate traps or drain pipes that he can use to drain all of the condensate that will be pushed through the system when the steam starts to flow. If there are heater valves built into your main stop valves, make use of them. Steam needs to be introduced slowly, and if there is confusion as to what is going on then you need to stop the job at once, and clear the confusion before anyone gets hurt.
In my experience, I find it helpful to meet face to face with everyone involved in a large or delicate operation. A person may tell you over the phone or walkie-talkie that they understand you, but you need to be able to see the expression on their face as well, to be fully confident that they do actually understand you. Everyone must also understand that there is one person who is the lead on the operation, and that he, she, or you as the case may be, must be made aware of any situations that arise, and make all relevant decisions that have an effect on the job as a whole. After you’re confident that everyone is on the same page, then you can start the job and talk remotely. During such operations, it is imperative that you keep all communications relevant and on point.
Another area of thermodynamics in pipefitting takes place when cold gases are introduced to warmer piping systems. This is why you might see frost on pipes where cold nitrogen is being fed from a pressurized tank into warm piping.
Many years ago, the Navy did some testing on emergency blow systems aboard submarines. This is the system the crew uses to get the sub to the surface in a hurry in the case of a catastrophic event. The high pressure air used for this purpose is pressurized in large air flasks, many of which are in the ballast tanks outside of the inner hull. When the system is activated, cold air from outside the hull rushes into the piping system within the hull and starts to condense and frost up. If the pipes in the system are not big enough to handle the condensing, then the frost will accumulate to the point of plugging the pipes solid, and the air will not reach the ballast tanks to blow the seawater out and raise the sub.
Finally we’ll touch on ambient air, or how the atmosphere surrounding pipes and components affects a system’s performance. We mentioned boiler tubes before. Boiler tubes must be constructed to handle immense heat, and even then, they will melt if the operator does not keep a flow of steam or water in the tubes at all times. This is why a continuous blow system is imperative when you are heating the boiler up, but before it is actually online sending steam into the system.
In cold climates outside water lines must be buried in the ground below the frost line, to ensure they don’t freeze up or become heavily insulated. There is a conception that when a freeze up does occur that the pipes burst when the ice heats up and expands. As often as not, what actually happens is water is trapped in the lines by closed valves or faucets. When the ice starts to form, the water trapped between the ice and the closed valve starts to increase in pressure from the ice, until it reaches a pressure too high for the system to handle, and the pipe bursts. It is imperative that any piping exposed to cold weather be insulated to aid in freeze protection. On a side note, this also helps in the warm weather as well, since the insulation helps cold water stay cooler in the warm weather, and therefore decreases the incidences of pipes sweating.
It is also a good practice to keep fuel oil lines insulated. When oil gets cold, it tends to thicken and flow slowly. If your plant has fuel oil stored in tanks, it is usually a good practice to periodically open drains at the bottom of the tank to drain off any water that may have collected due to condensate, leaks, or bad oil deliveries. If the water level reaches the bottom of the suction lines, you can suck oil into the lines along with the oil; and if the weather turned cold, the water could cause a freeze up which will cause your boiler to trip and cause heat losses and potential freeze ups to the building. If your tank does not have a drain system, then you should probably schedule periodic tank cleanings during the off season.
As a pipefitter, it’s easy to put all of your energy into ensuring that your systems can handle enough pressure, but you also need to be aware of anything else that could also have an effect on the systems that you work on.