The two primary causes of steam-trap failure are dirt, which can plug the trap or cause it to leak, and pressure surges, which can damage internal components. Both must be avoided in industrial process steam heating systems.
A steam trap’s function is to release excess condensate from steam piping to a drain or sewer. When steam is present, the steam trap closes to maintain pressure. Steam traps are fairly reliable, but they can fail open or closed.
If a steam trap fails open (figure 1), it dumps condensate and high-pressure steam to the drain or sewer, resulting in a loss of steam and increased energy costs to generate more steam. Excessive loss of steam can overload the boilers.
If a steam trap is plugged or fails closed, condensate will back up into the steam line and reduce the efficiency of the heat transfer process. This will deteriorate piping and adversely affect heat exchanger bundles, humidifiers and similar equipment. If the heat transfer process is compromised, it can cause a process shutdown until both the trap and any damage that may have been caused due to the failed trap are repaired.
Unless the steam trap is leaking, maintenance people have few clues that it has failed. If a condensate stack is installed, they might notice a steady plume coming out, but leaking and plumes are the only physical signs of a failure. Most steam trap problems are found through expensive manual audits, typically done annually by an outside service, and these audits miss many problems.
What is needed is a way to monitor all steam traps automatically. In many instances, conventional instrumentation is not practical because of the cost to wire the instruments back to the control-and-monitoring system. But, WirelessHART instruments make monitoring possible and affordable in a wider variety of applications, fueling their increasing use in this and other applications.
Common Problems with Industrial Steam Traps
Steam traps can fail open or closed (shut), causing a variety of problems. When a trap fails closed, a number of conditions can occur:
- Water Hammer. This is a condition where slugs of liquid become trapped between steam packets and accelerate to a high velocity. When accelerated, the slugs of water can create a hammer-like effect, causing extreme damage to plant equipment.
- Compromised Thermodynamic Efficiency. Water not removed from the steam system will collect in the low points of the system and plant equipment. One common place is in the heat exchangers. This buildup will cover the heat exchanger tubes, causing heat transfer to be compromised. Less heat transfer will have undesirable consequences for energy use, product quality and throughput.
- Water Impingement on Plant Equipment. If steam traps do not remove water from the steam system, droplets will be entrained in the steam. This entrained water can cause wear and tear on internal components of plant equipment, causing damage that may require expensive repairs and possibly placing plant personnel at risk. For example, water impingement can cause damage to turbine blades (figure 2).
- Pressure Surges/Steam Line Rupture. Condensate at saturation temperature is susceptible to flashing to steam if pressure in the system drops. Any valve opening has the potential to drop pressure, causing extreme pressure surges when the condensate flashes. This can lead to component and piping failure (figure 3).
When a steam trap fails in the open — or blow-by — condition, it constantly passes steam. Steam traps are built with an internal orifice to limit the amount of steam loss, but it can still be significant.
Steam traps installed on large, high-pressure steam lines can pass greater than 600 lbm/hr of steam. Depending on the plant’s cost of steam, this can cost upward of $30,000 per year.
As plants age, the number of steam leaks increases and plant efficiency decreases. Often, this increase in load is known as the phantom load. One executive estimated that 20 percent of his boiler steam production went to supply this phantom load, with a majority of it leaking through failed steam traps. Reducing steam loss through steam traps can reduce this phantom load and eliminate the need for capacity additions.
Wireless Process Monitoring of Industrial Steam Traps
One solution for many companies is to install wireless acoustic transmitters on their steam traps to continuously monitor their operation.
For example, one WirelessHART acoustic transmitter mounts on a pipe and listens for acoustic signatures between 25 and 45 Hz (figure 4). Acoustic signatures in this range mean a steam trap is open and releasing condensate. The wireless acoustic transmitters for monitoring steam-trap performance also have a temperature sensor that can detect cold or dropping temperatures from a clogged valve or steam trap.
The WirelessHART acoustic transmitter clamps onto a steam pipe for a simple, flexible installation. There is not a need to cut pipes or change pipe configurations. The wireless acoustic transmitter measures and sends the acoustic level — measured in the range of 25 to 45 Hz — and temperature — measured in the range of -40 to 500°F (40 to 260°C) — via WirelessHART to the plant’s control-and-monitoring system through a wireless mesh network.
Specialized software for monitoring steam-trap performance analyzes the signals from the wireless acoustic transmitters (figure 5). The software detects when a condensation release is occurring, when it stops and turbulence generated by a leaky steam trap. The software provides immediate notification of a failed steam trap.
A few brief case histories will demonstrate how the WirelessHART acoustic transmitters and software for monitoring steam-trap performance work together to identify failed traps.
Coffee Plant. At a coffee plant in Mexico, excessive steam-trap failures were reducing productivity in the plant by 20 percent per year because coffee production stopped during these failures. Maintenance was done by manual walk-arounds to check for leaks and failed steam traps, but the plant had no way of identifying which steam traps were failing more frequently than others. The plant had 100 steam traps, of which 60 were critical.
To better identify steam-trap failures, the plant installed 100 WirelessHART wireless acoustic transmitters on the 60 critical steam traps. Instruments from three steam systems connect to a distributed control system (DCS) via a smart wireless-mesh network. Each sensor transmits a WirelessHART signal, which is received — depending on distance — directly by a smart wireless gateway by one or more repeaters located around the plant. The plant estimates that detecting steam-trap failures and eliminating process shutdowns will increase production by 5,000 tons of coffee per year. Eliminating the need for manual walk-arounds will save $100,000 per year in maintenance costs.
Chemical Plant. A chemical plant had similar problems. They installed 90 wireless acoustic transmitters to monitor steam-trap performance and configured them to send data to the steam-trap monitoring software in only four days. After the installation, the system found eight failed steam traps. The return on investment was one and half months, with all savings from that point forward increasing plant profitability.
Food Processing Plant. At a food plant, the project engineer said, “We found 22 percent of our traps needed to be replaced during our last PM [preventive maintenance] check. By installing wireless acoustic transmitters, the plant will prevent steam loss with early detection of steam-trap failure. Not only will this minimize energy loss, but it will free up maintenance to focus their time and attention on things that need to be fixed to further improve our productivity.”
What Failed Steam Traps May Be Costing You
Steam systems are designed with steam traps to remove condensation from the piping to protect plant equipment and allow efficient operation of plant equipment and processes. When they fail, there are significant negative impacts.
The traditional method of checking those traps is to contract a third party to perform manual audits. These audits consist of measuring ultrasonic level and temperature at each steam trap to make a determination on the condition of the trap. This method has drawbacks in that it only looks at a short snapshot of steam trap operation. Therefore, it cannot always be a good predictor of trap condition because not all steam traps are required to operate 100 percent of the time. In addition, annual audits leave the plant operator susceptible to long periods of failed steam traps between inspections.
With the advent of wireless transmitter technology, continuously monitoring the health of steam traps is now cost effective. In order to implement a continuous-monitoring program, it is important to know where the largest impact is on a process. The factors deciding where the impact is most significant include both the size and failure rates of steam traps, and also their location in the plant and the important plant equipment they are protecting.
Adding continuous wireless monitoring of these critical steam traps typically will result in fast payback. In addition, the burdens of phantom loads, loss of steam and increased energy costs to generate more steam may be alleviated.
This article is the first part in a series from Emerson Process Management that discusses how WirelessHART can help monitor and control various processes in the facility. Look for the next installment on using WirelessHART technology for cooling tower control in an upcoming issue.