ELEMENT OF NGV

STATION

Importan element of NGV station

The stationary ngv station consists of :-

1. Compressor,

2. Dispenser,

3. Valves and piping system,

4. Storage system.

5. Control panel

COMPRESSOR

• used to draw gas from a distribution pipeline.

• compress it into storage system.

• Compressed gas dispensed to NGV vehicles (CNG tank)

TYPE OF COMPRESSOR

Two type of compressor used in NGV station.

1. Reciprocating compressor (widely used)

2. A screw type compressor is also been used

– Need lubricating system internally or externally– Need cooling system to dissipate heat quickly– Multi stage compressing system

Compressor Assemblies

• A compressor assembly includes– compressor, – Inter stage piping, – storage vessels, – pressure relief devices – control panel.

COMPRESSOR OPERATION

• The compressor uses suction to draw the CNG from a supply source and then transfers it through the station's pipelines and hoses under pressure.

• In some stations, the compressor discharges the CNG into a storage system for later use.

• In other stations it pumps it directly to the dispensing unit. Either a natural gas engine must drive all compressors or an explosion proof electric motor, approved for use in Class I installations (i.e. hazardous locations).

COMPRESSOR OPERATION

• A control panel is used to regulate the operation of the compressor.

• This panel should be installed in a remote operation booth.

• A manually operated shut-off switch must be installed in all CNG stations to quickly shut down the compressor during an emergency.

Compression System

• The system in his whole is made up of :• _ One part inside a 20” container that is divided in two

separate rooms:• · 1° room in which there aren’t pipes containing gas, and

that contains the electrical panel• (with plc), hydraulic power station (for box handling), and

pneumatic panel with air• compressor.• · 2° room containing compressor, booster, storage tanks

for high and medium pressure,• lubrication and cooling equipment.• Separation of the two rooms guarantees the physical

isolation of the components that work in

• contact with CNG (and so subject of ATEX normative) from those used for measurements and that

• have to be easy to get from the operator.• _ One part outside the container, in which there is the

connection to the distribution net.• Here gas is forced to pass through a capacity that helps

to damp pressure waves, and inside a• mechanical filter able to clean CNG from the impurities,

before getting inside the compressor.• The damping capacity and the contingent drying group

for CNG are normally optional and have to• be agreed with the customer.

• 1. ball valve with pneumatic actuator 2” (DN 50)• 2. ball valve 2” (DN 50)• 3. Filter F 1 mm DN 50 and contingent drying group (see enclosure 3)• 4. pipe 2” (DN 50)• 5. junction flange• 6. safety valve with connection ¾” M and 1” F ( calibrated 15 bar) + connection pipe

to vent• 7. damping capacity – condensate recovering• 8. ball valve ½” for condensate purging• 9. ball valve with pneumatic actuator 2” (DN 50)• 10. dilation joint 2” (DN 50)

FUNCTIONING SCHEME OF COMPRESSION SYSTEM

• CNG is compressed in four stages :• 3 stages of the compressor: pressure starts with 6 bar and gets 200

bar. • Compressed CNG is in part used to fill the medium pressure

storage tanks, in part sent to the fourth compression stage.• 1 stage of the booster: pressure gets 250 bar starting from 200 bar.

CNG is sent to the high pressure storage tanks.

• Two feeding lines of gas get the gas distributor:

1. CNG going out from the medium pressure storage tanks pass through a condensate separator (see enclosure 5) and goes directly to the Gas distributor2. CNG going out from the high pressure storage tanks, passes through a condensate separator

FUNCTIONING SYSTEM• and a pressure reducer (see enclosure 6), that brings pressure to 220 bar,

than passes through• another condensate separator and goes to the gas distributor.• SEE SCHEME 1• Along the CNG way there are pressure sensors, pressure switch, and safety

valves.• Pressure sensors give to the PLC the informations about gas pressure, so

PLC can use them to make• a careful and organic logic of control.• The use of pressure switch allows, in case of necessity, to stop

automatically the system when gas• gets a threshold pressure value. This value is calibrated differently for each

line of the circuit in• which the manostat is located (see enclosure 7).• Safety valves intervene sending CNG in a pipe connected to the vent, so to

the atmosphere, when it• gets too high pressures (more than the calibrated value). (see enclosure 8).

• W90 is an alternative compressor with the cinematic mechanism crankshaft - connecting rod –• crosshead – bar - piston; it is made of three cylinders single effect (one for each compression

stage).• Suction and exhaust of gas are obtained by a concentric valve with rings and plastic seal.• Suction valves can be open-locked with a pushing-valves moved by a pneumatic system that

allows• an idling start up of the electrical engine.• Many seals and scraper rings (to prevent the contact of lubrication oil with gas) work on the stem• that moves the pistons.• Between the seals there are many holes that allow purging to the vent the eventual lost gas.• Gas liners, lubrication oil, and CNG going out from each stage are cooled by a cooling water• circuit.• The compression cycle is managed by PLC thanks to the measure, made by the pressure

sensor, of• the pressure in the storage tanks.• Gas is taken from the damping capacity and sent inside the compressor room, using steel pipes

with• flanged joints.

• The connecting suction to the first stage is made by an assail compensator and a ball valve with

• pneumatic actuator.• At the source of the valve, gas pressure in the net is measured to verify that its value

is contained in• the functioning limits of the compressor.• CNG gets inside the first stage passing through the central section of the concentric

valve and goes• out through the external section, winning the force made by the springs and by the

pressure at the• discharge of the valve.• The temperature of the gas at the going out of the compression stage is measured by

a thermocouple• positioned on the pipe near the connection.• On a metallic block are positioned a pressure gauge, a pressure sensor, a calibrated

safety valve and• a discharge ball valve for purging the first stage.• M.T.M. S.r.l. - technical manual for CNG compression station -• 12

• On the outlet of the metallic block there is a ball valve with pneumatic actuator that gets the gas• before going in the heat exchanger water-CNG.• After having been cooled, gas is compressed in the second stage passing through a concentric

valve• similar to the one of the first stage.• At the outlet of the stage, gas temperature is measured by another thermocouple and CNG pass• through another metallic block similar to that of the first stage.• Ball valve with pneumatic actuator intercepts gas before the inlet in a heat exchanger water-

CNG.• After the heat exchanger gas passes in a condensate recovering equipped with ball valve for• condensate purging.• CNG is compressed in the third stage and makes the same circuit descript for the second one.• At the outlet of the condensate separator of the third stage, gas in sent to the medium pressure• storage tanks, passing through a metallic distributor block in which are mounted a check valve• (inlet), a pressure switch, a pressure gauge, a pressure sensor (for PLC management) and a ball• valve .

• Compressor is equipped by a forced lubrication system; oil is compressed by an electric engine

• powered pump. (see enclosure 9).• Oil is contained inside a carter realized in the compressor bed and, before

getting in the circuit, it• passes through a filter and a heat exchanger (see enclosure 10).• Oil pressure is 5 bar. A safety pressure switch stops system in case will get

under 2 bar.• Oil in pressure is sent to lubricate the compressor crankshaft, his own

bearings, the connecting rods• and the head cross of the three stages (as described in the scheme); good

lubrication of these• components is important to guarantee the correct functioning of the

compressor.• Check valves (positioned close to the components) and a pressure regulator

are used to maintain the• correct working oil pressure.

Compressor selection

• The size of compressor will depend on the quantity of gas required each hour and the overall gas requirement each day.

• The filling pattern and operating hours of the site are also important factors to bear in mind.

• Essentially you need to make sure that at any given time the vehicle trying to fill should be able to get a full (normally) 200 bar fill.

• This is a carefully worked out equation balanced between the size of the compressor and the size of the storage.

• Ideally you want the compressor (or compressors) to be running between 12 and 20 hours per day, this way you will maximise the gas compressed, and hence better your financial returns, but also give yourself time to take the compressors off-line for essential servicing or maintenance.

Storage cylinder and container

• Cylinders used for the shipping and storage of CNG must meet the design specifications established by the

DOT. • When bulk storage containers are used they must

conform to the design specifications of ASME or the DOT.

• Approved cylinders and containers are stamped or labeled with DOT or ASME approval markings.

• The certificate of fitness holder must make sure that CNG is dispensed in approved containers only.

• A cylinder rupture might occur when CNG is dispensed into a cylinder or container that does not have DOT or ASME certification.

Installation

• The safety distance required to conform with the Standard

Storage selection

• The design and specification of the storage cylinders is very much dependent on which country you will be operating in as each country has its own codes of practice and rules.

• When sizing the storage you will need to know how much gas is required and what compression top-up you will have from the compressors.

STORAGE TYPE

• For a fast fill system CNG pressure vessel

• Utilise 1 line and 3 line

• 1 line – low capacity

• 3 line – high capacity

CASCADE SYSTEM

Control system

• Priority sequencing system to divert the

• Main control system at control room

Priority control system

• Direct the filling onto the vehicle rather to the storage during filling

• No filling – direct the filling to the compressor

• Also control the three banks system (cascade system)

• Dispenser selection• Another important decision needs to be made on the type and

quantity of CNG dispensers.There are two main types, electronic and manual.

• • Electronic dispensers are the more sophisticated and have the

ability to be pressure and temperature compensated, that is they can dispense a very accurate quantity of gas no matter what the operation or ambient weather conditions.

•They also have the advantage of being “locked out” until some form of access card or point of sale equipment authorises it to be used.Electronic dispensers tend to be fully automatic in operation and are always found on public forecourts and large CNG stations.

•

DISPENSER• Manual dispensers are much simpler and have little or no electronics

inside them, instead they just rely on manual operation and simple manual valves. There is usually no means of monitoring or recording the quantity of gas dispensed and therefore these are mostly suited to small private stations such as CNG Forklifts or private car users.

• The quantity of dispensers required will very much depend on the base parameters of the station.This includes the number of vehicles using it per hour, the size of each fill, the type of vehicle using the station (cars, buses etc.) and the size of the refuelling station (compressor, storage, gas pressure)

• If for example it takes 5 minutes to fill a car (including time to attach and detach the filling hose) then you can only fill a maximum of 12 cars per hour on each hose.If you have 500 cars filling at a refuelling station then you will be need two hoses, i.e. 12 cars per hour for 24 hours = 288 cars per hose (12x24) per day, therefore need two hoses.In practice you will in this instance probably have two twin-hose dispensers (4 hoses in total) so that congestion at the pumps is keep to a minimum.

PIPING SYSTEM

• The design conform to ASME 31.3 Piping system for petrochemical plant

• Manual shut off valve

• Relief valve at appropriate location

• Automatic shut off valve

Pressure Relief Devices

• According to DOT regulations and the ASME unfired pressure vessel code, all CNG cylinders and storage

• tanks must have a pressure relief device installed.

Remote Manual Shut-off Switch

• A manual shut-off valve, designed to cut off the gas supply to the CNG station in case of an emergency,

• must be installed in the gas feed line. The certificate of fitness holder must make sure that the valve is

• protected against physical damage and kept accessible at all times. This valve is required in addition to any

• automatically operated shut-off valves. The distance between the remote shut-off valve and the compressor

• inlet, or any part of the storage system or dispenser system, is indicated in the table below.

• An automatic shut-off valve connected to the gas piping system will be installed on CNG stations to automatically cut off the CNG gas supply in case of an emergency.

• The valve must be positioned upstream of the confined high pressure gas piping.

• The valve must be installed underground or otherwise protected from exposure to fire and physical damage in a manner acceptable to the fire commissioner.

Automatic Shut-off Valve

Other safety device

• Gas detection system

• Heat detection system

Gas Detection Systems

• A combustible gas detection alarm system, meeting the standards established in the New York City Building

• Code, must be installed in all indoor CNG stations or compressor enclosures. This system is designed to

• automatically activate an audible and/or visual alarm when the CNG concentration in the atmosphere

• exceeds 20% of its lower explosive level (LEL). The system will automatically cut off the gas supply at

• 50% of the LEL and simultaneously transmit the alarm to the fire department via an approved Central Office

• Connection.

Heat Detection Systems

• Closed circuit detection devices designed to detect the heat generated by a fire must be installed in each outdoor CNG station, CNG building, or compressor enclosure.

• Heat detection systems for CNG buildings and compressor enclosures must automatically activate the facility's extinguishing systems and an audible and/or visual alarm when a fire is detected.

• They must also shut off the gas supply to the compressor and transmit an alarm to the fire department via a central monitoring station when a fire is detected.

Grounding

• All piping, compressors, storage systems, cylinders and containers installed on CNG stations must be electrically grounded.

• Vehicles with a cylinder or container installed must be grounded during dispensing operations. The electrical grounding reduces the likelihood of accidental ignition of the CNG due to sparks generated by static electricity build-up.

• Grounding may be accomplished by running a metal wire from the fill connection to an approved grounding source using an adjustable clamping device. For example, the

• equipment may be grounded to a 6 ft steel rod driven into the ground.

TERIMA KASIH