What are fixed orifice steam traps? Do they save energy when compared to conventional steam traps?
A fixed orifice steam trap continuously removes condensate from a steam line through a small orifice machined into an orifice plate. When condensate is present in the steam line, the trap allows it to pass into the condensate return system. When no condensate is present, the trap releases a small amount of live steam.
Continuous condensate removal allows the fixed orifice trap to be equipped with a smaller outlet—on the order of 1/8th to 1/30th the size of the outlet found in mechanical steam traps. Thus, the live steam loss through an orifice trap when no condensate is present is much smaller than the loss through a conventional trap that has failed in an open position.
Fixed Orifice Steam Trap Advantages and Disadvantages
Fixed orifice steam traps offer operational advantages and disadvantages. Advantages include:
- Live steam losses are limited due to the use of extremely small orifice sizes. Steam losses are also minimized as steam is blocked while condensate droplets are passing through the orifice.
- There are no moving parts to wear and replace.
- The traps can be mounted in any position.
- Air vents are not required.
- The fixed orifice traps are not sensitive to backpressure.
- The small physical size of fixed orifice traps reduces heat losses.
- Trap inventory requirements are minimized as a single trap body can be fitted with orifices of various sizes.
Disadvantages associated with fixed orifice steam traps include:
- A fine mesh strainer must be placed ahead of the fixed orifice steam trap as orifice holes are easily plugged with dirt, scale, or sediment, resulting in the backing up of condensate into the steam distribution system. The strainer must be equipped with a mesh size or opening smaller than the orifice. These strainers must be blown-down at least annually to avoid clogging. This is done by allowing steam to flow across the screen and with the steam and strainer debris released to atmosphere. Strainer cleaning requires maintenance time and commitment.
- Fixed orifices must be correctly sized or "engineered". A fixed orifice diameter can effectively drain condensate only within a limited range of discharge rates. If the condensate load varies, condensate may back up (causing corrosion, water hammer, or ineffective heat transfer). Oversizing the trap releases excessive amounts of live steam. Fixed orifice traps are suitable for variable flow loads (applications where the condensate load varies by a factor of over 4:1) when the steam flow is regulated by a modulating control valve. A new staged venturi orifice design is available that uses the flash steam emitted from the condensate as it discharges through the venturi to regulate flow by providing a local backpressure.
- A continuous back-up of condensate can create a water seal on the trap orifice and prevent the removal of non-condensable gases. Excessive corrosion can result.
- Erosive wear of the orifice hole due to the release of high velocity steam can lead to enlargement with a subsequent increased loss of live steam. Erosive wear does not occur when orifices are machined from stainless steel.
Do They Save Energy Compared to Conventional Steam Traps?
The answer to the energy savings question really is "it depends." In order for a fixed orifice steam trap to be effective, the condensate load must be relatively constant, the orifice must be sized correctly, the steam should be clean, pH must be controlled, and the strainer placed in front of the orifice must be periodically blown down.
While vendors of fixed orifice steam traps claim both maintenance and energy cost savings benefits, such benefits are difficult to quantify. Fixed orifice traps continuously release condensate and a small amount of steam, while steam losses from conventional traps include cycling losses and losses due to the percentage of traps that have failed in an open or partially open position. Potential energy savings are related to the difference between two unknown values: steam losses given existing system operation with current maintenance practices, minus expected steam losses after the orifice traps are installed and in use.
For instance, in "real world" operation, facilities have populations of properly functioning and of "failed" traps. A facility may have 90 fully functioning traps and 10 that have failed closed, open, or partially open. The percentage of failed traps found is likely related to steam pressure, trap type, and frequency of trap inspection and repair.
A correct comparison of a base or conventional trap scenario versus a fixed orifice scenario would compare the energy losses from the 90 properly functioning and 10 failed traps versus the steam losses due to using 100 orifice traps. The orifice in fixed orifice traps is small compared to the orifice in a conventional mechanical trap. Steam losses from even a relatively small number of failed conventional traps are substantial. It is this failed trap steam loss value that must be compared with the losses due to the use of orifice traps.
Key variables necessary to conduct an analysis include: the percentage of failed traps, the orifice size for those traps failing open or partially open, the mean time between trap failures, and the steam trap inspection frequency associated with the facility's maintenance program. After the losses per failed trap are determined, the last two terms can be used to determine the time (in hours per year) that a failed trap would be expected to release live steam before the next inspection/repair cycle.
Fixed orifice traps would likely save energy for systems with a long inspection/maintenance interval and with large numbers of failed conventional traps. Conversely, fixed orifice traps could release more live steam than an existing system where the percentage of failed traps is low. (Note: manufacturers of fixed orifice traps contend that steam losses through their traps are less than those associated with brand new, properly functioning mechanical traps.)
To overcome the limitations of installing fixed orifice traps in retrofit applications (where condensate loads are not known or may only be estimated), trap suppliers provide unions to facilitate the interchange of traps that can accommodate different size orifices. Optimum orifice sizing is determined by trial and error.
Another advance that is suitable for small condensate loads is the automatic variable orifice or thermal expansion steam trap. This trap cannot be oversized as its design incorporates a thermal modulator (a wax, plastic, or liquid) that has a high rate of expansion over a narrow temperature range. When condensate forms and cools, the modulator contracts, increasing the orifice diameter so the cooler condensate is released. The modulator expands and automatically reduces the orifice size as hotter temperature condensate, closer to the live steam, approaches.
Oland, C.B., Review of Orifice Plate Steam Traps, Oak Ridge National Laboratory, ORNL/TM-2000/353/R1, January 2001.
The DOE’s Best Practices Steam End User Training Guide, Section 9, 2010, describes multiple types of steam traps, including orifice traps, illustrating the differences and similarities among them.
Improving Steam System Performance:
A Sourcebook for Industry, 2nd Edition, DOE Energy Efficiency and Renewable Energy Advanced Manufacturing Office. DOE/GO-102012-3423. October 2012.
Summary of the Performance Analysis of Venturi Orifice Steam Traps , a Post Graduate Thesis by Shada Abu-Halimeh (Supervisor: Dr. Gavin Walker), Queen's University Belfast, February 2004.