District Regulating Station (DRS)

District Regulating Stations whose function is to reduce and regulate the pressure 19bar of gas supplied from the steel grid network into the Medium pressure PE network at 4bar. A DRS also filters any impurities such as dust that may be present in the gas. The principal components of a typical DRS are described as follows:

a) Inlet isolation: The inlet section to DRS comprises insulation joint & an isolation valve. The inlet thereafter bifurcates into two separate streams each provided with inlet as well outlet isolations.

b) Filtration: The filtration of incoming gas is through the filter of a rating adequate for the types of equipment installed downstream such as pressure regulators, relief valves, slams etc.

Types of Separators

Separators can be categorized into three basic types. These are vertical separator, horizontal separator, and spherical separator.

Vertical separator

The well stream is feed to the vertical separator tangentially through an inlet diverter that causes the primary separation by three simultaneous actions on the stream – gravity settling,centrifugation, and impingement of the fluids against the separator shell in a thin film.

Horizontal Separators

Horizontal separators may be of a single-tube or a double-tube design. In the single-tube horizontal separator, the well stream upon entering through the inlet strikes an angle baffle and then the separator shell. The liquid drains into the liquid accumulation section, via horizontal baffles

Spherical Separators

The spherical separator is designed to make optimum use of all the known means of gas and liquid separation such as gravity, low velocity, centrifugal force, and surface contact. An inlet flow diverter spreads the entering well stream tangentially against the separator wall. The liquid is split into two streams that come together after going halfway around the circular vessel wall and then fall into the liquid accumulation section. Liquid droplets from the gas are removed mostly by the velocity reduction imposed upon the gas inside the vessel. A mist extractor is used for the final removal of smaller liquid droplets in the gas.

 c) Slam Shut-off: Upstream of the pressure regulator is fitted the Over pressure shut off (OPSO) valves with the aim of shutting of the Gas flow in the event the outlet pressure exceeds the set values. OPSO protects the facilities & equipment downstream of the DRS. DRS is also fitted withUnder pressure slam shut off (UPSO) valves to protect the system in case the pressure drops to extremely low values (lower than the predefined values i.e. outside the safe operating envelope)

Overpressure protection

To prevent personal injury, equipment damage or leakage due to escaping gas or bursting of pressure-containing parts, it is necessary to install adequate overpressure protection when installing a pressure reducing regulator. Adequate overpressure protection should also be installed to protect all downstream equipment in the event of regulator failure. Some regulators are made with internal overpressure relief, whereas others require the installation of a separate relief valve or an additional regulator to act as a monitor. There are also shut-off devices that are designed specially to handle overpressure.

d) Pressure Regulating: Two pressure regulators are installed in series forming the “Monitor & Active” configuration with the one taking over the other in case of malfunction. This system ensures uninterrupted supply of Gas by automatic switching between the two regulators in the event of the “Active” suffering a malfunction. The models & ratings of both the regulators are the same with only a required variation in their pressure settings.

1. Direct-operated (Self- operated)

2. Pilot-operated

Direct-operated (Self-operated)

Direct-operated regulators are the simplest style of regulators. At low set pressures, typically below 1 psig (0.07 bar), they can have very accurate (+/-1%) control.. At high control pressures, up to 500 psig (34.5 bar), 10 to 20% control is typical. In operation, a direct-operated, pressure reducing regulator senses the downstream pressure through either internal pressure registration or an external control line. This downstream pressure opposes a spring which moves the diaphragm and valve plug to change the size of the flow path through the regulator.

Pilot-operated

Pilot-operated regulators are preferred for high rates or where precise pressure control is required. A popular type of pilot- operated system uses two-path control. In two-path control, the main valve diaphragm responds quickly to downstream pressure changes, causing an immediate correction in the main valve plug position. At the same time, the pilot- diaphragm diverts some of the reduced inlet pressure to the other side of the main valve diaphragm to control the final positioning of the main valve plug. Two-path control results in fast response and accurate control. In the evolution of pressure regulator designs, the shortcomings of the direct-operated regulator naturally led to attempts to improve accuracy and capacity.

•   Regulator Pilots
•   Set point
•   Spring Action

Regulator Pilots

The major function of the pilot is to increase regulator sensitivity. If it can sense a change in P2 and translate it into a larger change in PL, the regulator will be more responsive (sensitive) to changes in demand.

Set point

Set point and many performance variables are determined by the pilot. It senses P2 directly and will continue to make changes in PL on the main regulator until the system is in equilibrium. The main regulator is the "muscle" of the system, and may be used to control large flows and pressures.

Spring Action

Notice that the pilot uses a spring-open action as found in direct-operated regulators. The main regulator, shown in Figure 1, uses a spring-close action. The spring, rather than loading pressure, is used to achieve shutoff. Increasing PL from the pilot onto the main diaphragm opens the main regulator.

e)Pressure relief valves Inlet : A pressure relief valve limits pressure buildup (i.e, prevents overpressure) at its location in a pressure system. The relief valve opens to prevent a rise of inlet pressure in excess of a specified value. The pressure at which the relief valve begins to open pressure is the relief pressure setting. Pressure relief valves can be direct-operated or pilot- operated.

f) Safety Relieving:  A Safety Relief Valve is installed Downstream of the regulator for relieving of excess pressure build ups caused by the such things as the regulator valves not fully sealing at lockup, sudden drop of consumption or any other fluctuations causing an upsurge in pressure.

g) Gas Metering System : Gas metering is very important for transmission and distribution system and metering station.Throughout the world, gas measurement utilizes two basic principles to measure gas volumes, positive displacement meter and inferential meters. Positive displacement meters comprise the large majority of measurement devices in use while inferential meters are used primarily for large volume measurement.

Rotary Meter : 

This type of meter contains two oppositely rotating impellers, which are the measuring mechanism. The volume of gas is directly related to the number of revolutions of one of the impeller shafts. The rotary meter’s capacity rating is much greater than the diaphragm meter.It can be used in high pressure applications with up to an ANSI 600 rating available. These meters are compact and reliable, however since the operation depends on maintaining proper clearance between the impellers and the case, they can be susceptible to stress and if a malfunction occurs, then the gas flow could be stopped. The rotary meter is limited at high pressure. Therefore this meter although an excellent performance is not regarded as appropriate for large capacity, high pressure metering of natural gas.

Turbine Meters :

The turbine meter is classified as a rotary inferential meter. These flow meters are used successfully in both liquid and gas measurement. Turbine meters are velocity sensing meters with the volume of fluid being derived from the rotations of the turbine rotor. The speed of this rotor is proportional to flow rate. Turbine meters has been established as a means of measuring fluid for nearly 80 years. Since the 1950s, they have been considered favorably for the measurement of large volume gas flows. The designs have proved receivable, accurate and repeatable. As well as being used as the primary measurement standard, axial flow gas turbine meters are increasingly being used as calibration and reference meters.The turbine meter has wide range ability greater accuracy potential and more versatility in adding mechanical and electronic auxiliary devices.

Auxiliary devices Used with Meters:

The following auxiliary devices can be used with meters.

1) Electronic Volume Correctors :

The electronic volume corrector (EVC) accomplishes the same functions electronically as its mechanical counterparts. Because they are microprocessor based, they are more versatile (perform more tasks) and flexible (in date retrieval, manipulation and transmission) than mechanical devices. They are also less subject to accuracy loss due to vibration, wear and other mechanical failures. Features include:

· pressure and temperature correction

· calculated super compressibility factor using fixed gas quality values

· various volume outputs: uncorrected, corrected, totals

· imperial or metric unit choice

· built-in alarms indicating battery condition, pressure and temperature over under

   ranges, etc.

· telemetry capability (with data transmission devices added)

  EVCs are mounted above the meter's output drive shaft. They conserve battery power by

  remaining dormant between flow calculations, which are only performed on every

  complete revolution of this shaft. They are used on diaphragm, rotary and most turbine

  meters.

2) Flow Computers :

A flow computer has more program options than an EVC. Features include those listed

for EVC's with the following:

· Calculated super compressibility factor using full gas composition data (if available)

   Wide variety of alarm settings

· Most are not approved for use in hazardous areas

· Reprogrammable for other applications

· Performs some logic functions

· Calculated flows for several meter runs simultaneously

· Differential pressure (in mA) or pulse input accepted

h) Outlet Isolation: The outlet of the DRS is provided with an isolation valve through which Gas passes into the downstream Medium pressure PE network. Typical DRS internal layout and its installation at site are shown in picture below.

The main design parameter for a DRS is flow rate.

Actual flow rate of DRS (SCMH) = (Commercial demand at P1) + (Industrial demand at P2) + (Residential demand at P3)

where, P1-peak hour for the commercial segment,

P2- peak hour for the industrial segment,

P3- peak hour for the residential segment.

All demands are in SCMD.

Designed flow rate of DRS = (Actual flow rate ) * 1.3

Generally, DRS is available in the range of 1000 SCMH to 10000 SCMH. Based upon this flow rate we select the size of pipe, valves and fittings etc. All the DRS should be designed for two stream (1+1) one active + one standby basis.