The steam trap
Published: 08 April, 2015
The price of energy continues to cost industry dearly, but energy waste is costing even more. It’s little wonder therefore that sustainability has become such a hot topic. Understandably, much attention has been given to highly visible and energy hungry items such as motors, but in many instances the energy waste is invisible – in the form of lost steam. Even small leaks add up significantly over time. Hans de Kegel of AVT Reliability explains.
Around 22% of the steam that leaves boilers in a plant is lost via leaking traps, meaning predictive maintenance in this area is a worthwhile investment which can significantly reduce losses. In steam systems, a 3mm orifice under a pressure of 7 bar (100 psi) can result in the loss of 25 tonnes of steam per annum. If leaks are ignored, they can cause a drop in system pressure leading to lower operating efficiency, while over time the corrosive power of steam can cause equipment failure, resulting in expensive repairs or replacements. Then of course there is the risk of accident and potential liability. The Health & Safety Executive’s ‘Pressure Systems Safety Regulations 2000’ sets out its aim to prevent serious injury resulting from the failure of a pressure system or one of its component parts. This includes stringent guidelines covering the use of materials, the application of safety devices and the need for a suitable maintenance program. Planned maintenance can go a long way towards eliminating the damage caused by leaks, but at present there is no commonly accepted standard practice.
One of the common locations for leaks is in a steam trap, an essential component of any system. Because of their critical importance, it’s worth exploring this technology in more detail.
In a typical system, steam flows through a network of mains and branch pipes, all configured to deliver steam at specific points. The steam is propelled by differences in pressure and temperature caused by steam condensation. To control and eliminate this condensation from the system, steam traps are established at critical locations to purge condensate and other incondensable gases, allowing the steam to reach its destination in as dry a state as possible in order to perform its task efficiently and economically. The steam trap is therefore the most important link in the condensate loop.
There are three basic types of steam traps – all classified by International Standard ISO 6704:1982. Thermostatic traps react to changes in steam temperature, and will pass condensate when lower temperatures are reached. Mechanical traps – which include ball float and inverted bucket traps – sense and respond to changes in the difference in density between steam and condensate. Finally, thermodynamic (disc, impulse and labyrinth) steam traps are operated by changes in fluid dynamics and rely partly on the formation of flash steam from condensate. Combined with other variations – including continuous flow, intermittent or fixed – it is clear that there is a broad spread of technology available, each presenting unique challenges in terms of diagnosis and maintenance.
Typically, industry takes three broad approaches to aid accurate diagnosis of steam trap leaks, each of which can be highly effective in the right conditions:
The use of a testing-tee arrangement, test cock, or an inline sight glass for reviewing the steam trap discharge to the atmosphere can accurately determine: blow-by steam or a failed open condition; severe steam leakage; improper installation; under-sizing; incorrect type; and incorrect installation practice. This demands an in-depth understanding of the difference between flash steam and blow-by steam, but can expose the technician to the potential and damaging release of hot steam. Though a relatively low-cost option, there is a small additional cost associated with installing online visual testing components.
The pressure/temperature relationships of steam mean that temperature measurements can be extremely helpful in establishing existing steam system pressures, while thermal tests include Infrared Thermography, Contact Thermal Recorders (Thermocouples), and Infrared Point Radiometers (Pyrometer). Each has pros and cons. For example, while thermal testing devices will detect the temperature of the steam line ahead of the trap and at the discharge, they can only estimate existing steam pressure. Similarly, during infrared testing, the effect of the piping composition and/or insulation material on emissivity must be considered. So while measuring equipment is a vital weapon and must be an integral part of a steam system testing program, it cannot provide a definitive answer. It demands analysis and a true appreciation of the capabilities and limitations of the equipment.
Providing a versatile and accurate steam system diagnostic tool, ultrasonic testing allows the operator to hear sounds inaudible to the human ear. This type of equipment receives a high frequency signal (typically between 20-100 kHz) which is converted to an audible sound with the aid of headphones. Ultrasound devices that detect high-frequency sounds are a simple method of testing steam trap stations and are extremely accurate in detecting the distinctive high frequency noises made during the proper operation of a steam trap. The sensitivity of most high frequency monitoring equipment allows the testing person to hear not only completely failed steam traps (blowing steam), but even leaking steam from a trap in operation.
Knowing what to look for – and listen to – and what subtle variations mean will significantly reduce the chances of misdiagnosis, and so the experience of the technician is therefore of paramount importance in the process. For example, continuous flow traps should be discharging condensate continuously and an airborne ultrasonic analysis would expect to hear a modulating, continuous flow while infrared thermal inspection would indicate similar inlet and outlet temperatures. In the event of a blow-by condition, the acoustics would change from a modulating flow to an intense, continuous rushing sound with variations in the thermal conditions. In the event of a plugged trap, the acoustics would be quiet and significant thermal differences would be recorded between the inlet and outlet temperatures. This indicates the potential challenges facing accurate diagnosis.
The range of traps available, coupled with the sheer size of many steam operations and the complexities of detection, mean that it is not uncommon for leaks to be missed or misdiagnosed. This may lead to faulty traps being left in operation and/or functioning traps being replaced. The net result is the same: more lost energy and more unnecessary expense.
Surprisingly, perhaps, there is no commonly accepted model to address this challenge, but there are definite moves towards establishing strong and effective processes. While undertaking any form of survey is good, a best practice approach may be to take a more aggressive stance. In other words, rather than seeking to track down faulty leaks once they’ve happened, look to prevent them in the first place. Rather like insulating a loft to eliminate heat loss, such a commitment would deliver continuous and long-term return on investment. The key to this would be the introduction of a planned programme with a clearly defined schedule of steam trap survey routes – ideally on a quarterly basis. The routes would be generated in the site’s CMMS and be scheduled weekly.
In summary, steam leakage can have serious implications for industry in terms of financial and machine performance and health and safety risk. A proactive steam trap programme included within a steam system management programme has a huge impact on lowering energy costs, through identifying defective steam traps and calculating the loss each can be causing.
A planned and preventative maintenance programme can mitigate against these damaging effects, but to be truly effective it should be performed with a suite of technologies – visual, thermal and ultrasonic – and undertaken by experienced personnel. Under such a regime, significant long-term benefits can be achieved for the business, and for the environment.
For further information please visit: www.avtreliability.com