Three numbers decide the valve: set pressure from the maximum allowable working pressure of the protected component, required discharge capacity from the largest flow the pressure source can deliver, and nominal size from Durgo's blow-off capacity table at set pressure plus 10 percent. Get the first number wrong and the others are irrelevant.
Figures below come from Durgo's datasheets and product pages, and from the component definitions published by Spirax Sarco.
Step 1: Fix the set pressure from the component, not the pump
Four pressures decide the number, and enquiries routinely quote the wrong two.
- Normal working pressure (NWP): the pressure the system runs at under full load.
- Maximum allowable working pressure (MAWP): the design pressure of the weakest component in the protected envelope, such as a heat exchanger, a cylinder or a pipe.
- Maximum allowable accumulation pressure (MAAP): the highest pressure the system may reach, normally a percentage above the MAWP.
- Set pressure: the pressure at which the valve starts to lift. Durgo calls it the opening pressure and marks it on the valve.
Two rules pull against each other. The set pressure must be low enough that the relieving pressure, which is set pressure plus the overpressure allowed during discharge, never exceeds the MAAP, yet high enough above the normal working pressure that the valve reseats instead of weeping. It must never exceed the MAWP. Durgo publishes its capacity table at a 10 percent pressure increase, so 10 percent is the overpressure to carry into the check.
Worked example. A closed chilled water circuit runs at 0.6 MPa (6 bar). The weakest item is a plate heat exchanger rated MAWP 1.0 MPa, so with the common 10 percent accumulation allowance the MAAP is 1.1 MPa. Set pressure plus 10 percent must stay at or below 1.1 MPa, so the set pressure should not exceed 1.0 MPa. It must also clear 0.6 MPa with a surge margin, which 0.9 MPa does comfortably. Specify 0.9 MPa: relieving pressure is 0.99 MPa, inside the MAAP and below the MAWP. That value is also a row on the Durgo table.
Step 2: Size for the largest flow the source can deliver
The capacity figure is not the normal operating flow. It is the maximum flow that can be pushed into the envelope when the pressure control fails.
- A positive displacement pump delivers its rated flow against whatever pressure it can reach, so size the valve for the pump's full rated flow.
- A centrifugal pump delivers less flow as pressure rises, so take the flow from its curve at the relieving pressure.
- Thermal expansion is a smaller duty. A 200 litre cylinder heated from 10 to 60 °C expands by roughly 1.7 percent of its contents, about 3.4 litres, or 3.4 litres per hour over a one hour heat up. That is 0.06 litres per minute.
So capacity almost never governs a small water system relief valve: any Durgo DN15 body passes far more than 0.06 litres per minute. Capacity decides when the source is a pump, a compressor or a process feed.
Step 3: Read the nominal size off the capacity table
Durgo publishes blow-off capacity as litres of cold water per minute at a 10 percent pressure increase, without a discharge pipe. Read the row for the set pressure fixed in step 1, then move right until the figure exceeds the flow you need.
Worked example A, pump protection. A positive displacement pump moves 180 litres per minute, and step 1 gave a set pressure of 0.9 MPa. At 0.9 MPa the Durgo brass table gives 55 litres per minute on DN15, 144 on DN20 and 196 on DN25. DN20 falls short. Specify DN25.
Worked example B, overflow duty. A pressure maintenance set must pass 200 litres per minute at 0.6 MPa. The same table gives 161 litres per minute on DN25 and 280 on DN32. Specify DN32.
Worked example C, the compact range. A small oil line needs 60 litres per minute at 0.5 MPa. On the GT-77 table the 15/22 connection passes 47 litres per minute and the 20/22 connection passes 83. Specify the 20/22 version.
Two cautions. These are cold water tables, so air or steam needs its own calculation. The figures also assume no discharge pipe; fit one and capacity drops, because the pipe adds outlet back pressure.
Comparison table
All five are progressive valves, individually set on a calibrated test rig, sealed, and marked CE as category 4 equipment under Pressure Equipment Directive 2014/68/EU.
Which valve for which duty
- Closed heating or potable water circuit up to 125 °C and 0.5 to 2.0 MPa: the SV55, whose capacity table covers DN15 to DN50 in a single lookup. It is the default for most small and medium hydraulic systems.
- A standing overflow duty in the same circuit: the SV56. Same body, same table, same connections. When the valve passes a continuous overflow rather than answering an emergency, this is the part.
- Aggressive media, or anything above 125 °C: the SV65 or SV66 in stainless steel. The perfluor seal option reaches 220 °C, beyond the brass range.
- Small appliances, taps and compact oil or water lines below 1.0 MPa with 15/22 or 20/22 pipework: the SV77 (GT-77), the only model with a compression outlet and the shortest route to a set pressure in the 3 to 10 bar band.
- Where the real problem is backflow rather than overpressure: this is the wrong family. The backflow prevention range lists the vacuum and air admittance valves that address it.
Durgo's figures are specific to its own spring and cone geometry. Another maker's valve with the same DN and set pressure will not necessarily pass the same flow, so the capacity table and not the pipe size is the selection document.
Common sizing mistakes
- Reading the set pressure off the pump nameplate. It comes from the weakest protected component.
- Setting the valve too close to the working pressure. It lifts early, will not reseat cleanly, and drips permanently.
- Using the water capacity table for air or steam.
- Forgetting the discharge pipe. Every published figure assumes none.
- Choosing the nominal size from the pipe size. A DN25 line can carry a DN20 valve that cannot pass the required flow.
- Assuming a stock valve can be re-set on site. Durgo sets each valve to the ordered opening pressure, seals it, and marks the pressure and serial number on a red nameplate.
- Leaving off the handle or wheel. Durgo advises a control mechanism for regular checks, because in practice the checks are skipped and the cone sticks. These valves ship with a Teflon coated cone in a dry spring housing to reduce that risk.
- Checking the pressure rating but not the temperature. The brass range stops at 125 °C.
Frequently asked questions
How do I set the pressure on a Durgo pressure relief valve?
You do not. Durgo sets each valve to the requested opening pressure on a calibrated test rig, seals it, and marks the pressure and serial number on a red nameplate. The pressure is fixed at the time of order, and the available interval depends on model and nominal size.
What is the difference between a safety valve and an overflow valve in the Durgo range?
The duty, not the sizing arithmetic. Durgo publishes the 4155 safety valve and the 4156 overflow valve on one datasheet page with the same connections, pressure intervals and capacity table, and pairs the 4165 and 4166 the same way in stainless steel. A safety valve protects equipment against overpressure; an overflow valve holds a standing flow in a pressure maintenance system or pumping plant.
What is the difference between set pressure and relieving pressure?
Set pressure is where the valve starts to lift. Relieving pressure adds the overpressure allowed during discharge and is where the valve reaches full capacity. On these valves it is normally set pressure plus 10 percent, and it must stay inside the system's maximum allowable accumulation pressure.
Can I use the same capacity table for air or steam?
No. Durgo rates its safety valves for cold and hot water, air, oils and other media, but the published capacity figures are litres of cold water per minute. Air or steam needs a compressible fluid calculation.
What documentation should arrive with a Durgo safety valve?
A declaration of conformity, because the valves are CE marked as category 4 equipment under Pressure Equipment Directive 2014/68/EU, and Durgo states that a declaration is enclosed with all safety valves. An operation and care manual is published per model and seal compound. Brass bodies also carry a REACH note, since the brass contains more than 0.1 percent lead by weight.
Sourcing Durgo pressure relief valves
QINUO supplies genuine, brand new Durgo safety valves, overflow valves, vacuum valves and air admittance valves from its Hong Kong operation. Because the set pressure is fixed at the factory, an enquiry needs four lines: model, nominal size, set pressure, and the medium with its temperature.
Start from the brand directory or the Durgo brand page. The five relief models sit in the safety and overflow valve range, with the SV55, SV56, SV65, SV66 and SV77 pages carrying the individual data. The full product catalogue covers the rest of the fluid control range, and the application solutions section covers complete pressure protection packages. Send the four lines through the contact page.
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