Safety valve blowdown and boiler blowdown

"Blowdown" means two different things on the 4th Class exam, and questions often turn on which one is meant. Safety valve blowdown is a pressure difference: how far boiler pressure falls between the moment a safety valve pops and the moment it reseats. Boiler blowdown is a water treatment task: removing boiler water, and the impurities in it, through blowoff and blowdown lines. This page explains both.

Safety valve blowdown

A power boiler safety valve opens with a pop at its set pressure and stays open while boiler pressure falls. It recloses at a lower pressure. That difference, between the actual popping pressure and the actual reseating pressure, is the valve's blowdown. It's stated either in pressure units or as a percentage of the set pressure.

The blowdown is set by the blowdown adjusting ring. The ring controls how open the escape ports of the valve's huddling chamber are:

• With the ports fully open, steam leaves the chamber, the pressure under the valve lip stays lower, and the valve recloses after far less drop in boiler pressure.
• With the ports completely blocked, steam under the lip can't escape while the valve is open. Higher pressure builds under the lip, and boiler pressure must drop significantly before the valve recloses.
• Partly covering the ports gives a reclosing pressure between the two.

The ring changes the blowdown, not the popping pressure. Popping pressure is set by spring compression, and the fitted spring allows a change of no more than 5% from the marked setting. The ring is held by a locking screw that the jurisdictional Boiler Inspector secures with a wire seal.

ASME Section I valves must operate with a minimum blowdown of 2 psi (15 kPa) or 2% of the set pressure. Low-pressure steam heating boiler valves are different: they have no adjustable blowdown ring, and Section IV doesn't require adjustable blowdown on them.

Worked example: blowdown as a percentage

A safety valve is set at 1000 kPa. It actually pops at 1020 kPa and reseats at 970 kPa.

The blowdown is 50 kPa, or 5% of set pressure.

Three slips to avoid:

• Divide by the set pressure, not the reseating pressure. Dividing 50 by 970 gives about 5.15%, which is wrong.
• Start from the actual popping pressure, not the marked set pressure. The 30 kPa between the 1000 kPa setting and the 970 kPa reseat is not the blowdown.
• Don't add the overshoot. The 20 kPa by which the valve popped above its setting is already inside the 50 kPa difference.

Pass 4's home page has an interactive safety valve. Raise the boiler pressure and watch it pop, relieve and reseat, with the blowdown worked out as it goes.

Boiler blowdown

Boiler water picks up impurities from make-up water. Precipitates form faster when make-up water is of poor quality and when more of it is used. Blowoff and blowdown remove that water, and the impurities in it, from the boiler.

Blowoff. A blowoff is a valved connection in the external piping through which boiler water can be blown out under pressure. Drains on water columns, gauge glasses and feedwater regulators don't count. Bottom blowoff lines are in shock service. Blowoff sends short bursts of hot water down the line, so the piping needs room to expand and contract. Short surface blowoffs remove surface impurities, which upset the separation of steam from water and can cause foaming, priming and carryover. They continue until the boiling in the gauge glass settles down.

With two blowoff valves on a line, the valve nearer the boiler is opened first and closed last, so it works in still water and stays leak-tight. The farther valve does the throttling. Closing the farther valve first traps water between the two, and forcing the nearer valve closed against it can crack the valve body.

Continuous blowdown. Dissolved solids are most concentrated where steam first separates from water. A perforated continuous blowdown pipe sits at that level, about 15 cm below normal water level. Because this piping is normally hot and in service, it isn't in shock service, and it needs only one shut-off valve, placed as close as possible to the drum. An automatic system measures the conductivity of the blowdown water and adjusts a control valve to hold it at a set point. The heat in the blowdown water can be recovered into the make-up water, which carries it to the deaerator and then to the boiler.

Blowdown as a percentage. When blowdown isn't measured, it's taken as feedwater flow minus steam flow. The overall blowdown percentage is that difference divided by feedwater flow, multiplied by 100.

Which blowdown does the question mean?

• A question about a valve's set, popping or reseating pressure, the adjusting ring, or the huddling chamber means safety valve blowdown. That's in Part B, B3 Boiler Safety, CH01 Pressure Relief Valves.
• A question about removing water, impurities or dissolved solids, blowoff valves, blowoff tanks, or continuous blowdown means boiler blowdown. That's in Part A, A12 Boiler Systems, CH05 Blowoff and Blowdown Systems.
• The two percentages divide by different things. Safety valve blowdown divides by set pressure. Boiler blowdown divides by feedwater flow.

Practise both

A12 Boiler Systems is free, with a chapter quiz and podcast for Blowoff and Blowdown Systems. B3 Boiler Safety comes with a Part B plan.

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