Valve types, how they work, and what to put on the ends.

A flat wedge driven straight across the bore. Fully open it is a clear pipe with almost no obstruction, which is exactly what it is for and also why it is a poor throttling valve.

Isolation. Open or shut, nothing in between.

Quick opening — most of the area arrives in the first third of the lift

• Isolating a line for maintenance, where you want the lowest possible pressure drop when open
• Buried and underground water mains — resilient-seated gate valves are the AWWA standard there
• Large bore where a straight-through bore matters more than speed of operation
• Lines that are opened and closed rarely, and left one way for months

• Throttling. A partly-open wedge vibrates, and the seat erodes on the throttled side until the valve will no longer shut tight
• Anything that has to be operated quickly — a large gate valve takes many turns
• Slurries and solids, unless it is a knife gate; solids pack into the seat pocket and the wedge will not seat

A drilled sphere that turns a quarter turn between a clear bore and a solid wall. Nothing seals as reliably for as long with as little effort.

Fast, tight isolation. A full-bore ball valve open is very nearly a piece of pipe.

Roughly equal-percentage in a standard bore, near-linear with a V-port ball

• Anywhere a positive, bubble-tight shut-off is needed and it has to be quick
• Small and medium bore utility, chemical dosing lines, sample points, instrument isolation
• Duties that are operated often — the seats wipe clean every stroke
• Automated on/off service, where a quarter-turn actuator is the cheapest way to move a valve

• Throttling in a standard bore: the flow is forced through two crescent gaps that erode the seats, and once the seats are cut the valve no longer isolates — which was the reason to buy it
• Slurries with hard solids, which lodge in the body cavity behind the ball
• Duties where the trapped cavity between the seats matters — a ball valve shut on a hot liquid can over-pressure that cavity, which is what a vented ball or a double block and bleed is for

A disc on a shaft across the bore, turning a quarter turn to lie edge-on to the flow. The cheapest and lightest way to shut a large pipe, and the reason it dominates water treatment.

Isolation, and coarse throttling where the pressure drop is not critical.

Inherently close to equal-percentage; installed, it does almost all its work between 30 and 70 degrees

• Large diameter water and wastewater, where a gate valve of the same size would be enormous and cost many times more
• Where face-to-face length is tight — a wafer body is a few tens of millimetres between flanges
• Pump discharge isolation, filter inlet and outlet, basin isolation
• Coarse flow balancing, accepting that the useful range is the middle of the travel

• Fine throttling near the shut position, where the disc edge and the seat take the whole drop and the seat erodes
• Lines that must be pigged — the shaft and disc are always in the flow, even wide open
• Wafer bodies where the downstream side has to be dismantled while the line is live (see below)

A thin sharpened plate driven straight through the flow, cutting whatever is in the way. Built for the duties that stop an ordinary gate valve dead.

Isolation on sludge, slurry and fibrous solids.

Quick opening

• Sludge lines, thickener underflow, dewatering feed — anywhere a wedge would pack solid
• Screenings and rag-bearing flows, where the blade shears through the rag
• Powders and dry solids in hoppers and chutes
• Where a full round bore is wanted at a very short face-to-face length

• Bidirectional tight shut-off, unless it is specified for it — a plain knife gate seals in one direction and dribbles in the other
• High pressure — the body is thin and the blade unsupported across the bore
• Clean duties needing bubble-tight isolation, where a ball or resilient gate does it better

Flow turns up through a seat and a plug drops onto it. The Z-shaped path costs head even wide open, and buys you a valve that can sit half shut for years without damaging itself.

Throttling and regulation. The seat and plug are designed to be closed against flow.

Linear to equal-percentage depending on the plug profile — the trim is chosen for it

• Any duty where the valve is meant to sit part-open — bypass control, minimum flow, sampling
• Where a repeatable, adjustable flow matters more than pressure drop
• As the manual analogue of a control valve, and often the same body as one
• Frequent operation — the seat and plug meet perpendicular, so they do not scrape past each other

• Full-bore isolation duty where head loss matters — a globe valve wide open still costs several times the loss of a gate valve of the same size
• Large diameters: the body and the operating force both grow fast, and butterfly valves take over
• Lines that must drain completely, because of the internal weir

An elastomer sheet pressed down onto a weir in the body. The wetted parts are the lining and the diaphragm and nothing else — no stem into the fluid, no seat pocket, no cavity.

Isolation and throttling on aggressive, dirty or hygienic duties where nothing metallic may touch the fluid.

Roughly equal-percentage, with a usable throttling range

• Dosing lines: sodium hypochlorite, ferric chloride, caustic, acid — a lined body with an EPDM or PTFE diaphragm outlasts everything else
• Slurries and fibrous liquids that would jam a gate or a ball
• Hygienic and pharmaceutical duty, where the valve must drain and clean in place
• Where stem leakage to atmosphere is unacceptable — there is no path

• High pressure and high temperature: the diaphragm is the limit, typically well under 10 bar and 100 degC depending on the elastomer
• Duties needing full bore and low loss — the weir is permanently in the way
• Long service without inspection; the diaphragm is a consumable and it is the thing that fails

A tapered or eccentric plug that rotates a quarter turn. The eccentric pattern lifts the plug off its seat as it opens, so nothing rubs until the last few degrees of closing.

Isolation and throttling on dirty water and wastewater.

Near-linear over much of the travel with an eccentric plug

• Raw sewage, grit-bearing water, sludge — the classic wastewater isolation and control valve
• Duties where a butterfly disc would be fouled by rag but a knife gate is too leaky
• Where a valve must both throttle and shut tight on dirty service, which very few valves do well
• Buried service, because there is no cavity to fill with grit

• Very large diameters, where the operating torque becomes the limiting cost
• Duties needing minimum head loss wide open — the plug is a port, not a clear bore

A long tapered needle screwed into a small seat. Many turns of the handle move the flow area a little, which is the entire purpose.

Precise metering of small flows, and instrument isolation.

Linear and very fine — many turns for a small change in area

• Sample lines and analyser feeds, where a repeatable trickle is wanted
• Pressure gauge isolation and snubbing, to damp a pulsating reading
• Chemical metering trim, downstream of a dosing pump
• Anywhere a quarter-turn valve is simply too coarse

• Any real flow — a needle valve is a small-bore device and throttles everything, even wide open
• Dirty fluids: the annulus between needle and seat is narrow and blocks
• Isolation duty. It is a metering valve; it is not designed to seal repeatedly on the taper

A rubber sleeve squeezed shut from both sides. The only wetted part is the sleeve, and it is a straight, full-bore, unobstructed tube when open.

Isolation and control of abrasive slurries and solids-bearing flows.

Roughly equal-percentage; usable over most of the travel

• Abrasive slurries — lime, ash, grit, carbon, spent media — where a metal trim would be cut away
• Duties needing a full clear bore with nothing to catch on, including rag and fibre
• Dosing of settling slurries such as lime milk, where anything else silts up
• Where the valve must shut on a line carrying solids without trapping them in a seat

• High pressure and high temperature — the sleeve governs both, and it is rubber
• Hydrocarbons and solvents that attack the elastomer
• Duties where a torn sleeve cannot be tolerated; when it fails, it fails wide open or fully leaking

A hinged flap that forward flow pushes open and reverse flow slams shut. The simplest non-return there is, and the one most likely to cause a water hammer problem.

Preventing reverse flow. Nobody operates it; the line does.

Not applicable — a check valve is not a control device

• Pump discharge, to stop the line draining back through a stopped pump
• Where the head loss of a full-bore swing pattern matters and the line is horizontal
• Sewage and solids-bearing water, where a full-bore swing check passes what a dual-plate would jam on

• Pulsating flow, which makes the disc hammer against the stop until the hinge pin wears through
• Vertical downflow — a plain swing check will not shut against it
• Anywhere reverse flow can build up speed before the disc lands, unless the check is damped

Two half-discs on a central hinge, folded open by the flow and shut by a torsion spring. Short, light, and it closes before the column has time to reverse.

Non-return with a much lower slam risk than a swing check, in a wafer body.

Not applicable

• Pump discharge on clean water, especially where surge is a concern
• Where there is no room for a swing check body — this is a wafer, a fraction of the length
• Vertical lines, in either direction, because the spring does the closing rather than gravity
• Large diameter, where a swing disc would be too heavy to be quick

• Solids and stringy material, which catch on the central hinge and hold the plates open
• Very low flows that cannot fully open the plates — they flutter, and the hinge wears

A disc held on a seat by a spring. When the line reaches the set pressure it lifts, and it reseats when the pressure falls below it again.

Protecting a pipe, pump or vessel from over-pressure. The last line of defence.

Not applicable — it is a safety device, not a control valve

• On the discharge of any positive displacement pump, without exception — a PD pump against a shut valve will burst the weakest thing in the line
• On vessels and filters whose design pressure is below what the supply can deliver
• Thermal relief on a length of line that can be isolated full of liquid and then heated
• Anywhere a code or an insurer requires it, which is most places

• Using one as a back-pressure or control valve. It is sized to pass a fault flow, not to modulate
• Isolating one. A valve under a relief valve must be locked open or removed
• Discharging it somewhere unsafe — the relief has to go where the fluid can do no harm

A spring-loaded disc with a huddling chamber behind it. The instant it starts to lift, the escaping gas acts on a larger area and throws it wide open, so it pops rather than creeps.

Protecting against over-pressure on compressible fluids — steam, air, gas, vapour.

Not applicable — it is snap-acting by design, not modulating

• Steam and compressed air systems, where a slowly creeping relief would pass nothing useful and then fail to keep up
• Any vessel whose contents can flash or expand as vapour
• Where a code demands a certified, capacity-tested device rather than a general relief valve
• Where an audible warning that something has lifted is itself worth having

• Liquid service. The huddling chamber needs a compressible fluid to pop; on liquid it chatters, which destroys the seat in minutes
• Duties needing partial or modulating relief — that is a relief valve, or a control valve
• Discharging into a shared header without checking the built-up back pressure, which shifts the set point of every valve on it

A spring or a pilot loads a diaphragm against the upstream pressure. It opens only as far as it must to hold that pressure up, which is the mirror image of a pressure reducing valve.

Holding a minimum pressure upstream of itself, whatever the downstream is doing.

Modulating; it throttles continuously rather than snapping

• On the discharge of a metering or dosing pump, to give it a stable back pressure so the dose does not run away when the injection point is at low pressure
• To keep a minimum head on a distribution zone when a lower zone would otherwise drain it
• On a pump recirculation line, to keep the pump above its minimum continuous flow
• Ahead of a free discharge, to stop a line emptying and the pump running out on its curve

• Confusing it with a pressure reducing valve. A PRV controls what is downstream of it; this controls what is upstream. Fitted the wrong way round it does nothing useful at all
• Using it as a relief valve. It modulates and is not sized or certified for a fault flow
• Duties with solids, unless the trim is chosen for them — it spends its life part open

A spring holds it open and the downstream pressure, acting on a diaphragm, closes it. It settles wherever it must to keep that downstream pressure at the setting, whatever the upstream is doing.

Holding a constant pressure downstream of itself.

Modulating, and it throttles continuously by design

• Bringing a main down to what the downstream pipework, fittings and appliances are rated for
• District metered areas, where reducing pressure overnight is the cheapest leakage reduction there is
• Feeding a building or a skid from a main whose pressure varies through the day
• Protecting anything with a low rated pressure — plastic pipework, plate exchangers, membranes

• A large pressure ratio across a single valve. It cavitates, and cavitation destroys the seat and then the body. Two valves in series, or anti-cavitation trim, is the answer
• Zero-demand periods without thinking about creep: with no flow, a plain PRV lets the downstream drift up to the upstream. A small relief or a low-flow bypass valve holds the setting
• Confusing it with a back pressure valve. Same machine, opposite side sensed, opposite job
• Using it as an isolation valve. It is designed to sit part open and it will not shut tight