Minimum Operating Pressure Differential (MOPD) Explained
One of the most common reasons a solenoid valve "won't open" has nothing to do with the coil, the wiring or a fault at all. Instead, the valve is working exactly as designed – there simply isn't enough pressure across it to make it operate. This is the world of Minimum Operating Pressure Differential, usually shortened to MOPD, and understanding it is essential when specifying a pilot-operated valve. This guide explains what MOPD means, why it exists, and how to choose a valve that will actually open under your real-world conditions.
What is MOPD?
MOPD is the minimum difference in pressure between the inlet and outlet of a solenoid valve that is required for it to open reliably and stay open. It is a pressure differential, not an absolute pressure – what matters is the gap between the two sides of the valve, not the line pressure on its own.
Many pilot-operated (also called servo-assisted) valves have an MOPD of roughly 0.3 to 0.5 bar. If the differential across the valve falls below that figure, the valve may open sluggishly, only partially, or not at all. Manufacturers also quote a maximum operating pressure differential, which is the highest differential the valve can open against, but it is the minimum figure that catches most people out.
Why pilot-operated valves need a pressure differential
To understand MOPD you need to understand how a pilot-operated valve actually works. In a direct-acting valve, the coil moves the armature and that movement alone lifts the seal off the seat, so the valve can operate from zero pressure right up to its rated maximum. A pilot-operated valve is different: it uses the pressure of the media itself to do the heavy lifting.
When the coil energises, it opens a tiny pilot orifice in the diaphragm. Media then bleeds from the high-pressure side, the pressure above the diaphragm drops, and the line pressure underneath pushes the diaphragm up to open the main orifice. This clever design lets a small, low-power coil control a large orifice and high flow – but it only works if there is enough pressure difference across the valve to lift that diaphragm. Take the differential away and the diaphragm has nothing to push it open.
This is why a pilot-operated valve fed from a header tank a metre or two above it, or installed on a gravity-fed or recirculating loop with little pressure drop, can fail to open even though everything is wired correctly. The difference between direct-acting and pilot-operated valves is the single most important factor in whether MOPD will be a problem for you.
Direct-acting vs pilot-operated: MOPD at a glance
| Characteristic | Direct-acting valve | Pilot-operated (servo-assisted) valve |
|---|---|---|
| Minimum operating pressure differential | 0 bar – opens with no pressure | Typically ~0.3–0.5 bar required |
| How it opens | Coil lifts the seal directly | Line pressure lifts the diaphragm via a pilot orifice |
| Typical orifice / flow | Smaller orifice, lower flow | Larger orifice, higher flow for a given coil |
| Best suited to | Low or zero pressure, vacuum, gravity feed, on/off with little differential | Mains-pressure water, compressed air, systems with a reliable pressure drop |
| Coil power for a given size | Higher | Lower |
How to work out the differential in your system
The differential is simply the inlet pressure minus the outlet pressure at the moment the valve is asked to open. A few common situations are worth checking carefully:
Gravity or header-tank feeds. A column of water produces roughly 0.098 bar per metre of height, so a tank sitting three metres above the valve gives only about 0.29 bar – borderline or below the MOPD of many pilot valves. If the outlet is open to atmosphere this is your whole differential.
Recirculating and closed loops. In cooling loops, dosing skids and similar systems the pump may develop plenty of pressure, but if the valve sits between two points at almost the same pressure, the differential across it is small. It is the local drop that matters, not the pump head.
Discharging to atmosphere. When the outlet vents freely (filling a tank, a drain, a spray bar) the differential equals the inlet pressure, which usually makes MOPD easy to satisfy.
Back pressure. Long pipe runs, filters, nozzles or a second valve downstream raise the outlet pressure and shrink the differential. Always base your figure on the worst case, not the best.
Choosing the right valve for your pressure conditions
Once you know the lowest differential your valve will ever see, selection becomes straightforward. If that figure is comfortably above the valve's MOPD – for example mains-pressure water at 2 bar or more – a pilot-operated valve gives you high flow from a compact, cool-running, low-power coil. If your differential is low, unreliable, or genuinely zero, choose a direct-acting valve, which will open regardless of pressure. There are also assisted-lift (hung-diaphragm) valves that behave like a pilot valve at pressure but retain a mechanical link so they still open at zero differential, giving you the best of both worlds where flow demands are high but pressure cannot be guaranteed.
A quick sanity check when specifying: confirm the maximum pressure the valve must hold closed against, confirm the minimum differential available when it must open, and make sure both fall inside the valve's stated MOPD and maximum pressure ratings. Getting either wrong is a frequent cause of a valve that appears faulty on commissioning.
When a "faulty" valve is really an MOPD problem
If a newly installed valve hums or clicks but no flow follows, does not open on low-pressure or gravity systems, or only opens once the pump reaches full pressure, suspect MOPD before you strip the valve down. The coil is doing its job – the pilot mechanism just has nothing to work with. Our solenoid valve fault-finding guide walks through the wider range of symptoms and their causes if you want to rule out other issues as well.
Getting it right first time
MOPD is one of those specifications that is easy to overlook and expensive to get wrong, because the valve arrives, gets fitted, and only then reveals that it will not open under the actual site conditions. A few minutes spent confirming the differential at the point of installation will save a return and a re-order. For a broader walk-through of type, size, materials and function, see our guide on how to choose the right solenoid valve, or use the Valve Search Wizard to filter by pressure and application. If you are unsure whether a direct-acting or pilot-operated valve suits your pressures, contact our technical team with your inlet and outlet figures before you order and we will help you specify a valve that opens every time.