Pressure Reduction System

A Cost saving Arrangement for your Steam Processing Operation

Have you ever thought about why a technical consultant suggests buying

a high-pressure boiler? It then reduces the pressure by using the

pressure reduction valve (PRV) to the required level as per the process

requirement. In general observation, it seems like a cost

increasing step since you need to buy an extra device (Pressure

Reduction System) and other extra fittings to install in the steam line.

But it is not so!

Let’s dig a little deeper, whether using a PRS is practically feasible or not.

Steam generation at high pressure helps you to reduce the initial investment cost since at high pressure the specific volume

of steam is lower than that at low pressure, which requires a small diameter pipe to carry the steam and reduces the cost associated

with the pipe , which leads to a reduction in the initial investment cost. The high-pressure steam generation allows

relatively less carryover and provides better-quality steam, as at low pressure, carryover is more likely to occur. Using a high-pressure

boiler allows you to use the steam in different applications at different pressures by using the PRV. It also provides the

flexibility for variable

load. A sudden increase in load reduces the boiler pressure and increases the carry over and moisture content in the steam.

The PRV eliminates this and allows constant pressure to be maintained in the boiler. Furthermore, generating steam at high pressure

is relatively cheaper than using the low-pressure boiler, so it reduces fuel consumption.

Now we have a brief idea of what benefits PRS serves. Any porous plug or orifice valve which reduces the cross-section area across the flow,

works as a pressure reducer, but during the fluctuating loading, the outlet pressure of the valve will also fluctuate, which is not desirable,

so to eliminate the above issue, we use the PRV, which provides a constant pressure during the variable loading also.

Let’s look more closely at what the technical data suggests:

A pressure reducing valve is a regulating valve that maintains the steam pressure at the

outlet at a certain fixed, prescribed level that is lower than the inlet pressure. The PRV

not only reduces the pressure but also controls the flow by load fluctuation

dynamically. Therefore, using the change of state of steam caused by adiabatic steam expansion, not

only the pressure changes but the temperature, specific volume, and latent heat also change.

Effects caused by PRS:

1. Quality improved of steam: Pressure reduction is the use of adiabatic steam

expansion. Since the latent heat of steam changes as pressure changes, dryness

fraction increases.

For example: if the saturated steam at pressure is 10 kg/cm2 with a 95% dryness

fraction is reduced to 3 kg/cm2.

Enthalpy of steam at inlet of PRS = 181.2 + 0.95*481.8 = 638.91 kcal/kg

And the dryness fraction at the outlet,

= (638.91-133.4)/516.9 = 0.977

= 97.7%

Now we can see a 2.7% increment in the dryness fraction. which simply means less

condensate will form inside the pipe and more steam will reach the process.

  1. Reduction in piping cost. : As mentioned above, the PRV allows the
    generation of steam at high pressure, and at high pressure the density of steam is
    high, which requires less space, so high pressure steam can be carried in a small
    diameter pipe as compared to low pressure steam.
    Considering the above example,
    The density of steam at 10 kg/cm2 = 5.049 kg/m3
    Velocity of steam in pipe = 30 m/s
    Capacity of boiler = 2000 kg/hr
    Then, the pipe required to carry the high pressure (10 kg/cm2) steam at a rate of
    2000 kg/hr is,
    Steam capacity (kg/s) = density * area
    velocity 2000/3600 = 5.0490.785*D2 * 30
    Inner diameter D = 68.38 mm.
    And pipe required to carry the low pressure (3 kg/cm2) steam at the rate of 2000
    kg/hr is,
    The density of steam = 1.622 kg/m3
    Velocity of steam = 30 m/s
    2000/3600 = 1.622
    0.785D2 *30
    D = 120.4 mm
    So, by using the PRV before the process, a large piping coat can be saved since a lower
    diameter pipe will be required.

3. Reduced Carry Over: Low pressure boilers are more prone to carryover. The PRS

reduces the boiler carry over by allowing us to generate the steam at high pressure

and ensure the contaminants free steam reaches the process. Since carry over

reduces the life of the valves, strainers, and pipes, so PRS also improves the life of

the system indirectly.

  1. Even heating conditions: PRV keeps the outlet pressure constant and stabilizes
    the heating conditions, which ensures uniform productivity by maintaining the
    constant temperature.

  1. Single boiler for different applications: with the help of PRV, a single boiler of
    high pressure can be used for different applications at different pressures.

For example, if a steam process unit has three different operations in which

steam is required at 5 kg/cm2 pressure, 6 kg/cm2, and 4 kg/cm2 with a

capacity of 2 TPH, then in this case, instead of three different boilers, you

can choose a single boiler of 6 TPH capacity and pressure greater than 6


Precautions During PRV Installation: It is also important for you to know whether the

installation of PRV is appropriate or not. Since an improper location of PRV may cause you

losses and would not have any benefits. Some of the basic factors you should take into

consideration are as follows:

A PRS should be installed just before the process and not in the main steam line.

For larger pressure reduction, a combination of PRS should be used for the best and most controlled operation.

Have No Idea Where to start with?

We at THERMODYNE ENGINEERING SYSTEMS provide the most energy efficient steam energy solutions which are best suited to your process. Our team of experts is always at a phone call distance to help you make your process energy efficient and reduce the steam energy loss by optimising the operation. Contact Us to increase your boiler system efficiency and reduce the operating cost.