Plumbing notes

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These are primarily notes
It won't be complete in any sense.
It exists to contain fragments of useful information.

Pipe sizes and threads


This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

In europe / UK:

  • Imperial pipe sizes are to standards like BS 659.
  • metric pipe is to BS 2871
e.g. 15mm pipe has an outer diameter of 15.0 +- .045

Old 1/2" pipe were standardized on the bore (the inner diameter), and the outer diameter would depend on the type of pipe.

For example, gas pipes were initially defined by inner diameter, e.g. 1/2" was initially ~12.7mm (1/2") inner and ~21mm outer, but as materials evolved and the pipe didn't need to be so thick, so you would now typically find ~16mm inner and ~21mm outer. (So in this case the 1/2" doesn't actually refer to any physical size anymore, and why they seem larger than the size suggests.)

(Old 1/2"-bore water pipe might be ~1mm thick so be around 14.7mm outer. (which you can solder and probably compression-joint to modern 15mm metric pipe))(verify)

For similar reasons, the BSP thread size for 1/2" is ~20.96mm major (~18.63mm minor) and there's not a 12.7mm in sight

American standards tend to use inch fractions, as they do.

European standards (BSP, DIN, etc) tend to use metric - now, anyway. The UK used to use imperial and there are still pipes like that around.


This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Threads can differ in

  • Shape
Parallel pipe thread is the same width everywhere. A little easier to mass produce(verify), but imperfections (or differences) in the thread make it a little less likely to seal well without O-rings or similar.
Tapered thread, a.k.a. conical thread, becomes less wide throughout the thread, so has a different inner and outer diameter. Tapered is easier to seal because you can twist it until it wedges
  • Thread angle
  • Thread pitch - the distance between threads
  • Thread shape

BSP and NPT are incompatible because they vary in most things - shape and angle wouldn't necessarily be an issue if otherwise the same diameter (particularly with softer materials), but different pitch, and the parallel/taper difference is.

On low pressure systems you can sometimes still get away with it, but using the same system, or adapters, is generally better.

Roughly speaking there are two different systems covering most places:

  • BSP - British Standard Pipe
typically refers to parallel thread(verify)
55 degree thread angle

  • NPT - National Pipe Thread (American)
typically taper(verify)
60 degree thread angle

...but more variants exist:

  • BSP
    • BSPP: parallel (British Standard Pipe Thread Parallel)
    • BSPT: taper (British Standard Pipe Thread Tapered)

  • NPT - American Standard Pipe Taper Thread, 'for general use' (also known as ANSI/ASME B1.20.1)
  • NPSC - Straight Coupling Pipe Thread
  • NPTR - Taper Railing Pipe Thread
  • NPSM - Straight Mechanical Pipe Thread
  • NPSL - Straight Locknut Pipe Thread
  • NPTF - Pipe Thread Tapered (Dryseal)

And there are more standards, because of course there are

  • AN thread - Army-Navy, an american standard from WWI

NPS (Nominal Pipe Size)

Note that for common sizes, the difference between BSP and NPT is little, and that they will screw into each other for a number of revolutions, but not very many. After that they won't go further, you'll deform them - with a few materials this can make for a very tight fit, for most you'll just break them.

The amount isn't enough for serious work, but enough for some makeshift purposes.

There are also adapters.

British Standard Pipe (BSP) thread

  • 28 threads per inch
  • 19 threads per inch
  • 14 threads per inch
  • 11 threads per inch

America National Pipe Thread (NPT)

  • 27 threads per inch
    • 1/16 inch diameter
    • 1/8 inch diameter
  • 18 threads per inch
    • 1/4 inch diameter
    • 3/8 inch diameter
  • 14 threads per inch
    • 1/2 inch diameter
    • 3/4 inch diameter
  • 11.5 threads per inch
    • 1" inch diameter
    • 1 1/4" inch diameter
    • 1 1/2" inch diameter
    • 2" inch diameter
  • 8 threads per inch
    • 2½" inch diameter
    • 3" inch diameter
    • 4" inch diameter
    • 5" inch diameter
    • 6" inch diameter
    • 10" inch diameter
    • 12" inch diameter
    • 14" OD inch diameter
    • 16" OD inch diameter
    • 18" OD inch diameter
    • 20" OD inch diameter
    • 24" OD inch diameter


See also:


This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

A washtable tap might be made for a max of maybe 5 liter per minute at 1 bar to not waste more volume than you'll typically need, A kitchen tap might be made for 10 liter per minute at 1 bar, and a bathtub tap might be more than that.

Flow through a valve varies primarily with pressure, flow factor/coefficient of the valve itself, and specific gravity of your liquid -- see e.g. Flow coefficient, which tells you the amount of flow per pressure drop.

We usually deal with water, at 1 bar (approx 15psi), so it's mostly down to the valve.

A Kv (flow factor) of 1 indicates flow of 1 cubic meter per hour (m3/h) of water with an 1-bar pressure drop.

A Cv (flow coefficient) of 1 indicates flow of 1 gallon per minute for water (specific gravity of ~1) at 1 psi.

(With some extra things to plug into equasions, like that 1 m3/hour is about 16.6 liter/minute, and that 1 bar is 14.5 psi, it turns out that Kv is approx 1.16 times Cv (so Cv approx 0.86 times Kv), so if you ignore units they're on the same scale, and similar, which is sort of convenient. Some companies seem to use conversion factors nearer factor 1. Note that reported Cv/Kv testing methods/standards vary anyway so the conversion should generally be taken as a guideline, not precise. (verify))

Given typical household pressure, household taps seem to be made for a coefficient of approx 0.5.

A higher Cv can be useful e.g. in gravity fed cases, such as draining water from things like waterbutts.

For example, for a water height between 50cm (0.7psi) and 250cm (3.5psi):

  • a Cv of about 0.1 means ~0.02 - 0.05 liter/minute
  • a Cv of about 1 means ~0.2 - 0.5 liter/minute
  • a Cv of about 3 means ~0.6 - 1.5 liter/minute
  • a Cv of about 6 means ~1.3 - 3 liter/minute
  • a Cv of about 12 means ~2.6 - 6 liter/minute
  • a Cv of about 24 means ~5-12 liter/minute

See also:

2/2, 3/2, and more such indications

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Valves may have an indication like 2/2 or 3/2 (...which are probably the two most common types; there are more).

  • the first number is the number of ports on the valve
This counts input/supply, output/working, and exhaust ports
This typically does not count pilot channels (if any)
  • the second number is the amount of states
quite often 2
except for valves designed for some specific applications, and then rarely more than 3.


Two ports: typically used for a pressurized input, and output open Depending on the valve mechanism, it may be picky about which port is used as input and output.

Two states: flowing/open or not.

Applications include:

  • automated tap







Electronic valves


  • Electronic valves, particularly solenoid valves, are often meant to be used in one direction (usually indicated by an arrow) because various designs make it easier to hold back one way than the other ().
  • NO is Normally Open - acutuate to close (mostly applies to solenoid valves)
  • NC is Normally Closed - acutuate to open (mostly applies to solenoid valves)
  • Detented: will stay in its last state.
    • (In solenoid valves the word latching is regularly used)
    • May mean it can be operated both automatically and manually.

Solenoid valves

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Electronic valves often refer to solenoids valves, as seen in water intake for washing machines and dishwashers and such, valves for lawn irrigation, fuel valves, drink dispensers (though those may e.g. be peristaltic pumps instead), and more.

Driving and power

Solenoid valves are regularly AC, probably because various initial uses came from mechanical devices, and it require fewer parts to drive than DC.

DC valves are probably more interesting to digital-logic-controlled switching, but are less common.

Driving voltage is often

  • 24V (AC or DC) seems fairly common
    • irrigation valves are often 24V AC - 24V because it's safer than higher voltages, AC because it takes fewer components to drive.
  • 110/220 (AC) seems common inside some machines
    • washing machines often use these.
  • other voltages like 12V, 48V, sometimes lower voltages like 9V or 6V (then often DC)

Basic two-port valves are often either normally closed (and held open if power is applied), or less often normally open (closed when power is applied), which is mostly a choice of application, and what is sane when there's no power.

More-than-two-port valves often also have a natural state that a spring will return them to.

You also find latching valves, which will mechanically stay stably open and stably closed, and on which you use short pulses of current to switch. (Often need to be driven both ways on two wires, so something like an H-bridge (with flyback protection diodes) is one option)

Power draw: you can easily expect at least ~0.2A for small dispenser valves, perhaps 0.5A to 2A for household-use valves. Larger valves, say those with 2" and larger openings, may draw ≥10A.

The current during inrush often spikes higher (factor two-ish?) than the current used while the valve is held open.

Latching is more power-efficient for many applications since you only power the changes, not the holding. (Note that if leaving the tap open can be costly, If a fault or power outage is hard to rule out, a normally-closed valve is less risky.)

The solenoid can get hot to the touch, particularly if you leave it on for minutes or longer. Most valves are built to stand this heat.

Operation types

Note that many valve designs have a minimum input pressure below which they will not open. This is relevant when you want to dispense from things like non-pressurised bottles.

Many also have a maximum input pressure, beyond which they cannot overcome it to open (relates largely to the strength of the solenoid) or are likely to break.

Pilot operated (sometimes a name mentioning 'indirect') type valves largely leverage the pressure of the medium itself to open, by controlling a small pilot channel that causes a larger channel to open.

This design still needs some minimum (differential) pressure to open, typically on the order of at 0.3atm or more, varying with design and valve size / flow speed.

For example, for irrigation-type valves I've seen figures like:

  • minimum 0.2 bar, maximum ~5 bar, flow rate up to 40 l/min
  • minimum 0.5 bar, maximum ~15 bar, flow rate up to 100 l/min
  • minimum 1.5 bar, maximum ~20 bar, flow rate up to 150 l/min

Force pilot operated seems to refer to valves that have a pilot-channel design, but use it mostly to leverage more force, and which can be designed to work from zero pressure (so act more like direct-acting valves), but are not necessarily.(verify)

Direct acting valves usually work on low pressures differences. Some designs may still want 0.1 bar or so, some are zero-differential. If you want the latter, check the specs.

Gravity feed valves are used in situations where you have some non-pressurized source of liquid, meaning you'll want a zero-minimum-pressure valve. Consider things like laboratory setups, water butt irrigation, and such.

Particularly the lab variants are usually quite small, low-flow and exact, and more expensive, while for water-butt irrigation you may have flow of a few liters per minute up to a few dozen l/min for larger versions.

Seem somewhat unlikely to be latching(verify).

Assisted lift' - similar to direct acting, but seem to be higher-flow (verify)

Servo assisted (verify)

Smaller valves will often open and close within a dozen or a few dozen milliseconds, depending on size, pressure, type, and specific design.

Solenoid valve design means they are not meant for particularly large flows. For more flow, you would probably want a motorized valve instead. In comparison to solenoid valves:

  • motorized takes a lot longer to open and close (often on the order of seconds)
  • when they need to be open for minutes or longer, this is effectively a (slow) type of latching. (Which also means less safety against accidentally staying open, of course)

See also:

Further typology

If taste, sanitation or chemistry is important (which it isn't in, say, moving water for irrigation or washing machines), then consider the difference between diaphragm isolation valves and diaphragm actuated valves - the difference being in whether the fluid can reach the iron bits of the valve or not.

Motorized valves

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Valves with a motor (often a stepper motor or other high-torque design), and with the valve itself being low friction.

May have to be actuated both ways, or have a spring to return it when non-actuated. In the latter case they are functionally similar to (non-latching) solenoid valves (in that you need to apply power to keep them open).

Valve designs and types

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

(focusing those seen in plumbing, not those primarily seen in medical applications, music instruments or such)

Ball valve cross-section

ball valve

ceramic valve

  • a few designs
    • for small taps (e.g. sink faucets) it's usually the two-ceramic-disc design: disc have holes (very flat, and therefore watertight), one stationary, one rotated by the handle. Typically a quarter opens/closes, by aligning these holes.
    • ceramic ball valves also exist, as do a few related designs
  • very hard - does not wear as much as steel, does not corrode, does not rely on rubber seal, so lower-maintenance
  • interesting for use around corrosive liquids

Large butterfly valve

diaphragm valve

Wedge-type gate valve, shown closed.

gate valve

  • can be thought of as a pipe into which a plate/block/cylinder is lowered
  • change in flow fairly slowin reaction to operation (also an upside, in that it avoids fluid hammer)
  • when open, has low friction loss
  • many designs have small seepage when closed
  • can often stand very high pressures
Globe valve diagram (one of a few designs)

globe valve

Large knife-gate valve

knife gate valve

One design of needle valve

pinch valve

Simple and small laboratory-style plug valve (designs and sizes vary)

plug valve

See also:

Specific valves

  • Check valve / backflow preventer / directional control valve
Used whenever you want water to flow one way, usually because water can be dirty and should not flow back into the potable-water system.
There are e.g. often regulations for firehoses due to legionella. Automated refilling of central heating systems needs one of these.

Clogged things