The key to avoiding mistakes when selecting a steam trap orifice comes down to one thing: don’t decide by feel. Establish the actual operating pressure, the volume of condensate to be handled, and the start-up load as numerical values, then choose the orifice diameter and operating type that suit those conditions. Simply following this sequence prevents many problems. Conversely, if you decide on a model or size while the conditions remain vague, the likelihood of trouble such as steam leakage or poor discharge increases. This article organizes the selection criteria, the points people often stumble on in the field, and how to actually proceed, all from a practical standpoint.
Key points of this article
- Orifice selection starts by establishing three items as numerical values: “operating pressure,” “required condensate discharge capacity,” and “start-up load”
- Because the wrong size can lead to both steam leakage and poor discharge, avoid both over-allowing a margin and falling short
- Consider the installation environment as well (condensate pooling, debris), and settle any uncertain conditions through a selection consultation to be sure
Criteria for orifice selection
The three conditions to establish first
An orifice is determined by “how much condensate it can pass for a given differential pressure.” The starting points for selection are therefore the following three.
The first is operating pressure (the differential pressure between the inlet and outlet sides). Because catalog discharge capacities assume a differential pressure, the capacity will be misread if it does not match the actual operating conditions. The second is the required condensate discharge capacity. This is the amount estimated from the heating surface area and steam consumption, and it serves as the basis for the orifice diameter. The third is the load at start-up. Immediately after start-up, more condensate is produced than during steady operation, so how far you account for this peak changes the selection.
Symptoms caused by the wrong size
If the orifice is too small, condensate may not be fully discharged during start-up and other periods, remaining inside the equipment and becoming a contributing factor to reduced heating efficiency and water hammer. On the other hand, if it is too large, not only condensate but also live steam may pass through, potentially leading to losses from steam leakage.
It cannot be flatly stated that “selecting by feel will always turn out this way.” How the symptoms appear differs depending on the operating conditions and operating type, and whether poor discharge or steam leakage shows up is not uniform either—this is the important point. That is precisely why you need to make the judgment in accordance with the conditions, including how you factor in the margin.
Pitfalls of the installation environment and how to proceed
Attention to condensate pooling and debris
Even if the selection is appropriate, the trap cannot deliver its intended performance if piping layout causes condensate to pool upstream, or if scale and debris become caught in the orifice. Install the trap as low and as short a distance as possible from the outlet of the heating equipment, at a position where condensate naturally collects. Providing a strainer upstream and securing valves for inspection and blow-down helps reduce the risk of clogging.
How to actually proceed
The procedure is simple. First, write down the operating pressure, required condensate capacity, and start-up load. Next, choose the operating type (bucket type, disc type, float type, etc.) and orifice diameter that suit those conditions. Finally, check the installation position, strainer, and ease of inspection. These three steps let you avoid guesswork selection. If there are points where the figures are not yet firm, feel free to bring them to a consultation with those fields left blank.
Effects seen from diagnostic cases (results vary by condition)
At one chemical manufacturing plant, a diagnosis was performed on approximately 300 traps in operation, and the results showed the defect rate improving from about 40% to roughly 10–13%, with steam losses reduced accordingly. However, this is one example at one plant. At sites with a small number of traps, the percentage figures fluctuate more widely, giving them a different meaning than at sites with many. Results differ depending on equipment conditions, trap type, and operating conditions, and please understand that performing a diagnosis does not guarantee the same level of reduction in every case.
Frequently asked questions
Q1. Can I select based on the catalog discharge capacity as is?
A1. The discharge capacity is a guideline that assumes a specific differential pressure. Please make your judgment after converting it to match the actual operating pressure.
Q2. How much should I account for the start-up load?
A2. In general, more condensate is produced than during steady operation, but the multiplier varies with the equipment and how it is operated. The reliable approach is to organize the conditions and estimate on a case-by-case basis.
Q3. Is it safe to just choose a larger size to be on the safe side?
A3. If it is too large, it can lead to losses from steam leakage. We recommend the approach of selecting a diameter that is neither too large nor too small.
Q4. How do I choose between operating types?
A4. Each has strengths and weaknesses depending on condensate volume, pressure fluctuation, the presence of superheat, and so on. If you provide the application conditions, we can give you a rough guide for selection.
Q5. Are periodic diagnoses necessary?
A5. Orifice wear and clogging can progress over the years. The effect varies with site conditions, but it is an effective means of grasping the condition.
Q6. I don’t know the equivalent size of an existing unit.
A6. If the model number, engraved markings, and operating conditions are known, we can help with an equivalent selection. Please feel free to consult us with only the information you have.
Summary and consultation
Orifice selection begins with establishing the numerical values of operating pressure, required condensate capacity, and start-up load. Because both an oversized and an undersized orifice can lead to problems, the key is to decide with neither too much nor too little, in accordance with the conditions.
“I can’t judge whether the bore size is appropriate,” “I know the steam consumption but not how to estimate the condensate volume,” “I’m unsure how far to account for the start-up load,” “I want to select an equivalent for an existing trap”—if you have concerns like these, please let us know even just the conditions you have determined. Based on your application conditions, Yoshitake will provide a rough guide for selection and proposals for diagnosis. We also welcome consultations even when some fields are left blank.