PRESSURE REGULATING VALVES (PRV)

1. Objectives:

1.  State the purpose and applications of PRV.

2.  Explain the working of Bleed & Non-bleed type PRV.

3.  Perform maintenance of PRV

1.1   INTRODUCTION OF PRESSURE REGULATING VALAVE

The objective of a PRV is to provide a constant regulated desired output independent of the Variations in the supply side. Refer Figure-1. The objective of a PRV is to provide a constant Regulated desired output

As  an  electrical  generating  plant  supplies  voltage  at  a  much  greater  value  than  the  voltage rating  of  each  appliance,  similarly  plant  air  is  supplied  at  a  higher  value  than  the  supply Required by each individual instrument.  In  each  case  the  plant  supply  is  large  in  order  to Accommodate larger number of users. As voltage transformers step the voltage down before it is  used  by  the  appliance,  pressure  regulators  step  the  pressure  down  before  it  enters  the Pneumatic instruments. Now the question arises that why pressure Regulation is necessary? For any kind of air – actuated equipment there is usually one pressure which will provide the most efficient operation. At a higher pressure the equipment will wear excessively with no Significant increase in output, and compressed air will be washed.

Compressed air costs money. Total cost of compressed air can be cut by regulating the line Pressure to the working pressure recommended for the equipment in use. In choosing the air pressure regulator, attention must be paid to the Regulation characteristic and  the  Flow  characteristics.  The  first  determines  the  ability  of  the  pressure  regulator  to maintain a constant and even delivery pressure as selected, independent of the fluctuations in the primary pressure. The closer this control the better the regulator. Flow  characteristics  demonstrate  the  ability  of  the  regulator  to  maintain  its  secondary  or delivery  pressure  under  varying  air  demand.  Here  again  the  regulator  which  provides  the most even pressure will be the most desirable.

1.2 TYPE OF PRV’S

Generally there are two varieties of PRV’S viz.

Non Bleed Type

Bleed Type

Let us first discuss the operation of a simple Non-Bleed Type of PRV.

1.3 PRINCIPLE OF OPERATION OF A NON-BLEED TYPE PRV

A simplified schematic of a Non-Bleed type of PRV is given in Figure-. Here the desired output air pressure is balanced against the characteristics of a spring. High-pressure input air is connected at the left. Regulated air at a reasonably constant pressure issues from the right. Air will pass through the device only when the pressure of the spring is great enough to force the diaphragm downward. As the diaphragm moves downward it pushes the small pilot valve down,  permitting  air  to  flow  through  the  main  air  passage  to  the  output.  The  output  air pressure  appears  in  the  space  under  the  diaphragm.  Output  air  pressure  pushes  up  on  the diaphragm against spring tension. When the output air pressure is great enough, the upward force will overcome spring tension and the diaphragm will rise. The pilot valve will rise with the  diaphragm  and  shutoff  any  further  flow  of  air. 

If  the  demand  for  the  air  increases  the reduced pressure in the output line will again allow the calibrating spring to force the pilot value open.  The output air pressure will again be built up until spring pressure is overcome. It is evident that the tension of the spring will determine the operating level. Spring tension is adjusted by turning the compression screw with  the  know  at  the  top  of  the  regulator.  Refer Figure- for a typical Non-Bleed type PRV (More Model). A small hole, called as ‘Breathing Hole’ is provided in Bonnet. The purpose of this hole is to provide a uniform atmospheric pressure in between diaphragm and adjusting screw space. In absence  of  this  hole,  diaphragm  would  experience  a  variable  force  on  top,  subject  to  the position of diaphragm.

1.4 PRINCIPLE OF OPERATION OF BLEED-TYPE PRV:

Figure  shows  a  simplified  schematic  of  a  Bleed  type  of  PRV.  There  is  no  connection between the pilot valve stem and the diaphragm. The rounded end of the valve stem acts as a second  valve.  With  its  seat  fastened  at  the  center  of  the  diaphragm.  If  the  output  pressure should become too high, the extra force will  lift the diaphragm from the valve stem, allowing excess pressure to bleed into the spring chamber which is open to atmosphere. For instance, assume that the regulator has been set to operate at 20  psi. As pressure was applied to the spring, the pilot valve was pushed down and air flowed from input to output.

The  pressure  gauge  in  the  output  line  is  necessary  to  tell  us  when  the  required  pressure  is reached, assuming the regulator is not calibrated. Assuming that air is not being consumed, at 20 psi output air pressure the diaphragm will have forced the calibrating spring upward so that the pilot valve just seats itself and no air can flow. Suppose we now want to change the output air pressure to 17 psi. The tension on the spring is relieved a little by turning the knob at the top of the regulator.  The excess pressure under the diaphragm now has sufficient force to lift it. The valve stem cannot go further up because it is hitting against its seat. The diaphragm air will bleed off to atmosphere. Generally  PRV  is  used  in  conjunction  with  a  filter  and  a  drip  well.  Oil  and  water  are deposited on the filter surface and ultimately drain into the well where they can be removed by opening the drain cock.

1.4.1 Maintenance of PRV

PROCEDURE: –

Physical inspection. Check for any physical damage and missing parts/ screw etc.

Dismantling :-

Reduce  the  spring  tension  of  Pressure  regulating  valve  by  rotating  adjusting  knob  anti-clockwise  with  proper  size  of  spanner.  Open  the  bonnet  screw  with  proper  size  of screwdriver one by one diagonally put the all parts in a container, spring, spring seat, bonnet and diaphragm. Open the bottoms cap Allen key screw by the proper size of Allen key, diagonally and put in The container.  Open the filter post by proper size of screwdriver.  Open the plug assembly With proper size of spanner and put the all parts in container.

Inspection of parts:-

Inspection of the diaphragm, pilot for hard and cracks and puncher by testing with hand.

Servicing & cleaning: –

Clean all the parts of Pressure regulating valve by proper detergent CCL4 or petrol using soft brush. Clean the filter by petrol and back wash method, remove the particle from the filter.

Replace of parts: –

Change the damage/ bad part by proper size of spanner.

Greasing – Apply the grease for spring seat and diaphragm.

Re- assembly:-

Re-assembly the plug, plug spring and part in the body using proper tools.

Assemble the filter parts.

Assemble the bottom cap putting gasket and Allen screw tight diagonally.

Put the diaphragm, spring, spring seat and bonnet and screw, tight the screw in diagonally.

Put the spring tension-adjusting nut.

Put the output pressure gauge.

Put the input & output filters.

1.5 TESTING OF PRV

For testing of a PRV we will adopt the following steps: Change the number of loads. PRV output should remain constant. Change the output of main PRV. Output of PRV (under test) should remain constant.

PRV-1 is for varying the testing PRV-2 supply pressure.

PRV-2 is under testing.

PG-1 for supply pressure measurement.

PG-2 for output pressure measurement.

Load variation valves are provided.

PROCEDURE:-

Connect the set up as shown in figure. Give the air supply to PRV-1 and set out of PRV-1  to 40 PSI. Set the PRV-2 output 20 PSI. Vary the PRV-1 output and observer the PRV-2 output. Vary the PRV-2 load and observer the output.

1.6 AIR FILTERS

INTRODUCTION:

Why should we use a filer? Air is naturally wet. So saturated compressed air, when cooled, will  release  liquid  water  which  must  be  removed  from  the  compressed  air  system.  Air is Dirty. It is polluted by fumes and contains dust, pipe scale, joining compound and quite often oil carry over from the compressor. Fumes mix with water forming weak acids or electrolytes Causing galvanic corrosion.  The compressed air filter should obviously remove from the Airline solid materialized,  such  as  pipe  scale,  dirt  etc.  Which will accumulate  because  of Corrosion inside the pipe and because of dirt entering from the atmosphere. The filter should also be capable of removing liquids such as water and oil, which may be carried along by the compressed air.  The  restricted  passage  of  controllers  are  easily  plugged,  making  dirt  and fluids the big maintenance problem on such lines.

SELECTION CRITERIA

In choosing a filter the three important points are: Efficient removal of dirt, water etc. To  effect  its  removal  with  the  least  possible  pressure  drop,  all  extra  pressure  drop  being  a Further waste of air. To provide simple draining and cleaning.

GENERAL DESCRIPTION AND WORKING

Refer Figure – for the operation of a simple Air Line Filters (Norgren model).Filters remove Solid and liquid contaminants from compressed air lines. To remove the liquids and solids, the air enters the bowl through direction louvers and sets up a centrifugal flow pattern that forces the liquid particles to the inside wall of the filter bowl. From here, liquids run down into the “Quiet Zone “ in the  bottom  of  the bowl where they are trapped by a baffle to prevent air turbulence from carrying them back into the air stream. Accumulated liquids and solids are blown out of the bowl by simply opening the drain. All air leaving the bowl on the way to the pipeline must pass through the filter  element  which removes the  solids  remaining  in  the  air.  Automatic-drain models drain collected  liquid Automatically.

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