Electrically Actuated Plastic Valves

Electrically Actuated Plastic Valves

Electrically actuated valves are an increasing requirement for plastic pipe systems, due to demands for electrical efficiency and 24/7 system operation.

Types of electrically actuated valves

Electrically actuated valves are available in three standard configurations;

·         Open / close

o   Power source is required to open and close the valve e.g. when power is applied to terminal 1 and taken away from terminal 2 the valve will open and the opposite will occur when the power is switched across. Open / close type valves are suitable for automated filling and emptying of tanks, reservoirs. They are also very useful in fully automated processes like backwashing filters and priming pump supply lines prior to an automated pump staring. Open / close type valves are often used at the end of plastic pipe lines, where a line is required to be purged before refilling once a pump starts.

·         Spring Return

o   These are available in either a normally open or normally closed configuration. When normally open the plastic valve will, with the aid of a powerful internal spring, return to the open position when the power is removed. When normally closed, the valve will close when the power is removed. Spring return valves are often used for emergency supply lines, where, in the event of power failure a bypass needs to be opened, a line needs to be drained or a tank needs to be filled or drained.

·         Modulating

o   Modulating valves have the power constantly applied to them and have an additional controlled power source of 0-10volts dc or 4-20mA. The controlled power source is supplied by a controller that is usually responding to input from a sensor, such a s a pressure or flow sensor. The controlled power source partially opens and closes the valve to give a controlled flow. The accuracy of a modulating valve will only ever be as good as that of the sensor and controller.

Modulating valves are excellent at controlling tank levels by using feedback from either a level sensor or a sensitive, submerged pressure sensor. Other applications include controlling the pressure in a supply line, although this is more often done using an inverter (speed controller) if it is a 3 phase pump that is supplying the line.

Technical Bulletin: Plastic pipe threaded fittings

1. Thread types

Plastic pipe systems such as PVC and ABS have the advantage of being able to be quickly and easily assembled, using solvent cement that is painted on the pipe and socket. There may however be occasions when a threaded fitting is required. Such a situation often occurs when connecting to pumps or other equipment or when transitioning from one material to another (such as plastic to metal).

BSP Threads

Nearly all metric and most European made imperial threaded pvc pipe fittings, have a BSP Parallel thread on them. The “BSP” denotes that the set and depth of the thread conform to “British Standard Pipe” thread. An additional “P” is often used as a suffix (BSPP), to denote the thread is a parallel one rather than a tapered one. Additional suffixes of either “M” or “F” are also sometimes used. These denote whether the thread is a male (i.e. thread on the outside) or female (i.e. thread on the inside) fittings.

NPT Threads

Apart from BSP, the other commonly found thread in plastic pipe systems is NPT, an American standard (National Pipe Taper), which is only usually found on plastic pipe parts that originate in the USA.

Comparison of BSPP and NPT threads

BSPP and NPT threads are generally not compatible. The first hurdle we come to is that one is parallel and the other tapered (a tapered thread is a conical shape, whereas a parallel is the same diameter all the way along). As such, even if the pitch and thread depth were the same, a male parallel will only go into a female tapered by a single thread and will not be secure. A male taper will usually go into a female parallel about 40-50% of the way and can, as long as a thread sealant is correctly used, be regarded as being generally secure. 

Pipe Size	Pitch  (Threads/Inch)
	NPT/NPS	BSP
1/16"	27	---
1/8"	27	28
1/4"	18	19
3/8"	18	19
1/2"	14	14
5/8"	---	14
3/4"	14	14
1"	11 1/2	11
1 1/4"	11 1/2	11
1 1/2"	11 1/2	11
2"	11 1/2	11
2 1/2"	8	11
3"	8	11
3 1/2"	8	11
4"	8	11
5"	8	11
6"	8	11
8"	8	---

The above table gives the pitch (the number of threads per inch) of NPT and BSPP threads. As can be seen, the only two sizes that are compatible are ½” and ¾”. All other sizes have different pitches and so will thread together. The ½” and ¾” sizes will connect, but only if the male part is NPT and the female is BSPP, and then only expect the thread to insert to about 40-50% of the whole thread.

Connecting different thread types

The preferred option to connect different thread types is to use one or more adaptor fittings. For example if it is required to connect a 1” BSPPM (male) to a 1” NPTF (female), the correct method would be to us a 1” BSPPF : Plain socket, and a 1” NPTM : Plain nipple. This would extend the length of the fitting, which in some cases may cause issues, however it ensures that a system is correctly fitted, and suitable for its design pressure specification.