Raise the Pressure to 145 psig With One Unmarked Safety Valve
A 500 hp watertube boiler is stamped MAWP 150 psig and carries two safety valves. Valve A's nameplate is legible: 150 psig set pressure with a certified capacity, current test certificate in the file. Valve B's nameplate is corroded and unreadable, there is no traceable National Board number on the tag, and the only test certificate in the file is 11 years old. A process change needs 145 psig at the header. The boiler has been running at 120 psig. Tonight the plant manager tells you to raise the pressure control set point to 145 psig and "sort the paperwork out later." The boiler also has a high-pressure limit switch set at 150 psig and a modulating pressure control.
1. Can you raise the set point tonight? State the governing rule you are applying.
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No - not on the information available.
The relevant principle is that a boiler may operate up to a pressure only if its certified relieving capacity is adequate at that pressure. Concretely:
- Safety valves are set at or below the MAWP, and the total rated capacity of all valves must be enough to relieve the boiler's maximum steaming capacity without letting pressure rise more than the permitted accumulation above MAWP (typically 10% for steam service).
- That capacity is not a guess - it comes from the valve's certified capacity at its set pressure, traceable to the manufacturer's or National Board's data.
- Valve B has neither a known set pressure nor a known capacity. You cannot credit it toward the required relieving capacity. If Valve A alone cannot relieve the boiler at full firing rate, then at 145 psig the boiler may have insufficient relief capacity - and at 145 psig with only 5 psi to the set point, there is no margin for a control overshoot.
The operating pressure is therefore capped by what the certified relief can protect, not by what the process wants or by what the stamp says. The high-pressure limit switch does not substitute for relief capacity; it is a control, and controls fail.
2. What exactly must happen before the boiler can legally run at 145 psig, and who does it?
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Valve B has to become a known quantity:
- Remove Valve B and send it to an approved safety valve repair facility (or replace it). The shop sets it to the required pressure, tests it for popping and reseating, verifies capacity from manufacturer's data, and issues a test certificate with the set pressure, capacity, and date, plus a seal and tag.
- The valve's identity must be traceable - National Board number or manufacturer's model and orifice designation. An unmarked valve cannot be certified, and without capacity data it is scrap.
- Reinstall correctly: upright, unobstructed, drainable, no shutoff valve between the boiler and the valve, and the discharge piped to a safe location.
- Verify total certified capacity against the boiler's maximum steaming capacity at the intended firing rate - not the current firing rate.
- Confirm the supporting instruments: the pressure gauge accurate and in range, the high-pressure limit set below the lowest valve set pressure, and the modulating control tuned so it does not overshoot into the valves.
- Update the boiler records and make the notification the jurisdiction requires; some states require notification or a new certificate when relief settings change.
The operator does not set, adjust, gag, or wire down a safety valve. That work belongs to a qualified valve shop and is documented.
3. The process needs steam tonight. What are your options, and what do you do with the manager's instruction?
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Options that don't compromise the relief protection:
- Operate at the pressure the certified relief can actually protect. If Valve A alone has adequate capacity for the boiler's maximum firing rate, 145 psig may be permissible once the controls and limit switch are verified - and that becomes a documented engineering decision, not an operator's guess.
- If Valve A alone cannot carry the boiler, cap the firing rate at a level Valve A can relieve, and accept the reduced steam output. Reduced capacity is a business problem; an unprotected boiler is a life-safety problem.
- Attack the demand side: reduce process load, shift it to another boiler, use the pressure-reducing station differently, or run a temporary line. 25 psi is usually a piping or demand problem, not an excuse to run past proven protection.
- Expedite the valve work - a valve shop can often turn a set-and-test around in a day. Start that call tonight.
With the manager's instruction:
- I do not raise the set point. The instruction is not one I can carry out, and I say so plainly and without drama.
- I record the request and my refusal in the boiler log with the reason: unverified relief capacity on Valve B. That log entry is the record that the decision was made knowingly.
- I put the deficiency in writing to the manager and the owner, and notify the jurisdiction if the boiler is ordered into service anyway.
- I do not gag, block, or "temporarily" disable Valve B, and I do not accept the argument that it "probably" relieves at 150 because it lifted last year. A safety valve that has not been tested is not a safety valve - it is a piece of brass with a history.