There are four types of drain valve and there are pros can cons for each.

Manual Drain Valve

Manual Drain Valve

You can use a hand operated valve – ball valve, needle or gate valve or other type of valve.  Simply go round regularly and open the valve to let the water out, closing it again when air comes out instead of water.Pros – The most inexpensive way in terms of components to drain water from your compressed air system Cons – Probably costs more than you think in terms of the time it takes each day or week. Will you remember? What happens during holidays or sickness? There is also a large potential cost if you forget.

Mechanical Drain Valve

Mechanical Drain Valve

This type of drain valve has a float. When the water rises above a certain level the drain opens and lets the water out.Pros – Cost. Prices range from inexpensive to very expensive. Good option areas in which electrical items are restricted or prohibited due to an explosion risk. Also a solution if there is no electric supply near by. Does not waste any compressed air. Cons – Maintenance issues. If there is dirt or contamination in the air supply it can interfere with the float mechanism and cause the drain not to open or to get stuck open.

Timed Drain Valve

Timed Drain Valve

This type has a solenoid valve and a timer combined into one. The opening time and the interval time can both be set. For example it can be set to open for 2 seconds every 45 minutes. When buying a timed drain valve it is important to check the orifice size – a larger orifice will be more reliable as it is less likely to get blocked by contamination. Valves with larger orifice tend to cost more.They often have a strainer fitted immediately upstream. It is better if you can managed without a strainer as this becomes a maintenance item by itself.  Rather fit a valve with a larger orifice and no strainer. Pros – Very simple and normally reliable. Can be inexpensive compared to other types of automatic drain and avoids issues with air locks if the drain point is very near the ground. Cons – Wastes compressed air. As you need to set the times for the worst conditions, the drain time will be generally longer than it needs to be so some compressed air will be exhausted as well as water. Over a 12 month period the cost of this lost air can be considerable.  There are also maintenance issues with stainers and contamination and there is no easy way to monitor the operation of the drain or feedback data to building management systems.

Electronic Level Actuated Drain Valve

Electronic Level Drain Valve

At the heart of this item is a capacitive sensor with two set points. When the liquid level reaches the top point a valve opens and lets the liquid drain out until it reaches the bottom point when the valve closes again.This type of valve often contains electronics which can send a signal to a building management system to report that a fault has occurred. Pros – It does not waste any compressed air and is normally good at handling contaminated condensate.  It also has the possibility to be connected to a building management system.  Probably the most cost effective way to drain condensate overall in the long run Cons – Initial Cost.

Well, you can also get a pressure switch which gives a pneumatic output.

Pneumatic Pressure Switch

This is a piloted three port, 2 position valve (3/2) with an adjustable set point. Available normally open or normally closed.

Suitable for a wide range of applications including those where electrical signals are not permitted, atex, explosion proof or intrinsically safe applications. Can also be used for pneumatic systems where introduction of electric controls would not be required otherwise.

That is why you get dew on the grass in the morning. On a still clear evening warm air (from the daytime) comes into contact with the ground which is cooling down. This cools the air down and then some water condenses into liquid form.

What happens in a compressed air system is similar to this. Ambient air (including water vapour) is compressed which raises its temperature (due to the law of physics). The high temperature air can hold this amount of water in the vapour form. Then the air leaves the compressor and enters the pipework system and cools down. This causes water to condense out.

It is this liquid water that causes damage to pneumatic components and air tools.

So, if your compressor is making water it is not the fault of the compressor! It is due to the laws of physics plain and simple.

The amount of water that results will vary with atmospheric conditions.

There are many different ways of dealing with this water problem. Some more and some less expensive. The correct one for you often depends on the application.

Some of these drying methods include:

Application dryer (Eliminizer)
Membrane dryer
Fridge dryer
Dessicant dryer

Air Receiver

A receiver (or air tank) is a steel vessel which stores compressed air. Because air is compressible a receiver stores a large amount of energy and it must therefore be manufactured in accordance with guidelines laid down.

Most air compressors require a receiver in order to operate correctly and efficiently.

To size an air receiver, find out the free air delivery of your compressor in litres/second (use our conversion tables if your compressor output is given in other units e.g. cubic ft/min (cfm) or cubic metres/min (m3/m) etc).

The receiver size (in litres) should be between 6 and 10 times the compressor output.

For example:

The output of a 18.5KW screw compressor is approximately 100 cubic ft/min (cfm) at 7 bar which translates to 47 litres/second.

47 x 6 = 282
47 x 10 = 470

The ideal receiver size therefore is between 282 and 470 litres. A receiver should rather be slightly oversized than undersized.

This guide only applies if the receiver is solely used to allow the compressor to work correctly. There are applications where receivers are installed for other reasons (e.g. as a backup air supply) in which case a completely different calculation would be used to determine the receiver size.

Oscillating Valve

This valve assembly will, when connected to a pneumatic cylinder, cause the cylinder will go in and out indefinitely until the air supply is removed.

The speed of both the in and out strokes can be controlled and can be  different from each other.

It is made from standard components and works by detecting the pressure in the exhaust side of the cylinder. When there is no pressure the cylinder is at the end of its stroke (or has got stuck in mid position) and can therefore be sent in the other direction. It continues in this manner until the air supply is removed.

The oscillating valve shown here is 1/4″ BSP but the principle can be applied to larger or smaller valves which are available on request (using standard components which are normally available from stock).

Step 1 – Turn off all your machines that use air.
Step 2 – Run your compressor till it stops and note the pressure in the receiver.
Step 3 – Start your stopwatch
Step 4 – turn off your compressor
Step 5 – time how long it takes for the pressure to fall by 1 bar (time = T in seconds)

To calculate, proceed as follows:

V = Receiver voume in litres
H = Hours per week that your factory or workshop is operational
E = Electric costs in £ per KWH

V x 60
———– = leak rate in cubic metres / min (LR)
T x 1000

A screw compressor will (typically) deliver approximately 0.149 cubic metres/minute free air at 8 bar per KW.

KW required to maintain leak = LR/0.149

Cost per year = KW x E x H x 52

This in only an approximate calculation but is sufficiently accurate for all practical purposes in industry and can be used for any size of factory.

Sample Calculation

Air receiver is 500 litres and it took 45 seconds for the pressure to fall from 7 to 6 bar.
Electric cost is £0.15 per KWH and the shift pattern is 16 hours a day, 5 days per week = 80 hours per week.

500 x 60
————- = 0.667 cubic metres / minute (Leak Rate = LR)
45 x 1000

Cost per year = (0.667/0.149) x 0.15 x 80 x 52 = £2,793 in electricity costs alone.

In addition to the electricity cost is the servicing and capital cost of the compressor although this is typically somewhat less that the electricity costs (estimate 20-25% of the electricity cost).

Do not tolerate compressed air leaks of any kind!!

Bottom & Back Entry Pressure Gauge

Bottom entry or back entry
The connection is either at the bottom of the gauge or on the back. Which is correct for you depends entirely on the application and where you are using the gauge. Panel mounted gauges are always back entry.

Pressure range
What pressure do you want to measure? Gauges should not be used continuously above 75% of their maximum scale reading. So, if you want to read a pressure of about 10 bar then use a 0-21 bar gauge or for 6 bar use the 0-11 bar etc.

Dial size
How large a gauge do you require? Larger gauges can be read from a greater distance (obviously). Standard dial sizes are 40mm, 50mm, 63mm & 100mm.

Scale
What units does the gauge read in? Many gauges read in bar and psi (pounds per square inch). Gauges for very low pressures have scales in millibars. Gauges can be made to have any scale that you require and red & green zones (for example). This is normally not economical for small quantities.

Glycerine filled or dry
Gauges can be filled with glycerine. The main purpose of this is to eliminate vibration. The machine that the gauge is fitted to may vibrating or there may be rapid fluctuations in pressure, both of which can make it difficult to read the gauge. Glycerine filled guages are very widely used in hydraulics.

Gauge and absolute pressure
These terms are sometimes used in connection with gauges. A gauge that measures ‘Gauge’ pressure is measuring the pressure above atmospheric pressure. So, when it is disconnected (or open to atmosphere) it will read 0. An absolute pressure gauge measures pressure compared to a total vacuum. This type of gauge, when it is disconnected (or open to atmosphere) will read approximately 1 bar (depending on the atmospheric pressure on the day). In industry 99% of gauges measure ‘gauge’ pressure.

Vacuum and pressure
A pressure gauge normally reads zero with the needle pointing the bottom left (about 7 o’clock). A vacuum gauge works on the same principle as a pressure gauge but when it is reading zero the needle points to the bottomr right (about 5 o’clock).

Orientation
More of an issue than you might think. When you fit the gauge and tighten it up will it be facing in the right direction? Some gauges have taper threads and it is possible to stop tightening in the right position and still achieve a seal. For parallel threaded gauges the use of a gauge adaptor is recommended. This adaptor has a male and the swivel female thread – the adaptor can be fitted to the connection point and then the gauge held in the correct position and the female thread tightened up against it.

Compound gauge
A compound gauge is one that measures both vacuum and pressure. At zero pressure the needle points to about 10 o’clock. Under vacuum the needle moves anti clockwise from that position and under pressure it moves in a clockwise direction.

Medium
What are you measuring the pressure of? You must make sure that the internals of the gauge are compatible with the medium whose pressure you are measuring. With compressed air or water and hydraulic oil this is not normally an issue. If you are working with explosive, flammable, toxic or oxidizing materials proceed with extreme caution and seek advice.

What are ‘Wetted parts’?
The parts on the inside of the gauge that come into contact with the medium (what you are measuring the pressure of). Even if the medium is compressed air or other type of gas they are still referred to as the ‘wetted parts’!

Medium temperature
In general medium temperatures above 65 deg C should not be used on standard gauges.

Steam, oxygen and acetylene
Special gauges designed for the purpose must be used for these media.

Pressure gauges is a very large subject and this is only intended to cover some of the more basic points – for further information or advice refer to your distributor, manufacturer or instrument specialist.

Male Thread

Female Thread

Sealing Point

A taper thread seals when the two threads come in contact with each other and requires ptfe tape or thread sealant to work effectively.  A parallel thread seals on the flat surface at the edge of the female thread and normally has a built in O ring of washer to make the seal.

Assembly Height

Parallel threads provide a fixed assembly height where as with taper threads, the assembly height depends on how tight you do it up.

What Fits What?

Both Taper and Parallel male threads can be fitted to parallel female threads but only taper male threads can be fitted to taper female threads. A Parallel male thread will NOT fit a taper female thread.

Which to Use?

Most BSP female threads are parallel.  Parallel threads provide more reliable seals and, in our experience, should always be considered in preference to taper threads if possible.

Taper threads wear out if connections have to be remade repeatedly and in rare circumstances a female thread can be damaged by overtightening a male taper thread.

If the surface at the edge of the female thread is not flat or uniform then a parallel thread will not seal and a taper thread must be used.  Also, if you are unsure of the application then a taper thread will work in more situations than a parallel thread.

The coupling and plug must be from the same series to connected together and each have three options: a female thread, a male thread or a hose tail (or barb).

Single/Double Shut Off

‘Single Shut Off’ means the downstream side (plug side) is vented to atmosphere when the coupling and plug are separated. Normal for compressed air couplings.

‘Double Shut Off’ means both sides (coupling & plug) are sealed when the coupling and plug are separated. Normal for hydraulic couplings.

Safety couplings

When a single shut off coupling and plug are separated the air on the downstream side is vented to atmosphere. This release of energy can make the hose whip and cause injury. To avoid this ‘safety’ couplings should be used. Most new installations are now fitted with safety couplings as standard. There are many different types of safety coupling but almost all work by making the disconnection a two stage process. The first stage exhausts the air on the downstream side. The second stage allows the coupling and plug to be separated. The coupling and plug cannot be separated until the air has been exhausted.

How do you identify a coupling?

If there is no part number on the coupling half, compare the profile of the plug to a picture or a shadow chart. Some plugs have a distinct profile and are easy to identify but others look similar and are more difficult.

What type of coupling should you use?

The correct coupling for you depends on a number of factors including:

• What type is already in service in your factory?
• What flow rate do you require?

The most commonly used couplings are:

	PCL Standard (or Series 19)Long established couplings widely used in industry in the UK
	Schrader Standard (or Series 17)Long established couplings widely used in industry in the UK
	ISOB6 (or Series 23)Medium flow rate (1250 l/min), widely used in Europe – good choice for a new factory in the UK.		Series 25Higher flow rate than Series 23 (1800 l/min) – also very widely used in UK and Europe.

Fortunately, there is a simple test you can carry out yourself to determine the approximate output of your compressor and this may then allow you to look elsewhere for your issues.

Simple load test procedure is as follows:

1. Start with the receiver completely empty of air – i.e. 0 pressure on the gauge
2. Close the isolating valve to the system
3. Turn on the compressor and start your stopwatch
4. Note time (T) in seconds taken till the compressor goes off load and what pressure it achieves (P2) in bar.
5. Note the size of the receiver (V2) in litres.

Calculation is as follows:

P2 x V2
———    =    Compressor output in litres/second
      T

1 litre/second = 2.119 cfm

As a rule of thumb the output of a compressor (at 7 bar) can be related to the HP or KW of its motor.

Screw or Vane compressors – 1HP = 1.89 l/s or 1KW = 2.53 l/s
Piston (or reciprocating compressors) – 1HP = 1.42 l/s or 1KW = 1.89 l/s