Fri. Sep 24th, 2021

City gas distribution

Transporting future fuel

#76 Cascade & Priority Panel

18 min read
  1. CNG cascade is an arrangement of cylinders which acts like a buffer vessel in which natural gas is stored at high pressure & then is used for dispensing.
  2. Cascade can be stationary or mobile. Cascade is divided in three banks based on pressure viz. High bank, Medium bank, & Low bank.
  3. There are two types of cascades which need to be mentioned, viz. stationary cascade & mobile cascade. Stationary cascade are installed at CNG station, which are used for minimizing the stop/start of the compressor. The CNG is dispensed from the storage system into cylinders mounted on the motor vehicle. Mobile cascades are the cascades mounted on heavy commercial vehicles HCV or light commercial vehicle LCV. It is used for transportation of gas from mother to daughter stations.
  4. The cascade comprises of a number of freestanding or horizontal high-pressure seamless gas cylinders for CNG gas service suitable for a filling pressure of 250 bar at 15 degrees Celsius.
  5. The cylinder is mounted in the frame such that there is no lateral or rotational movement of the cylinder during transportation.
  6. The cylinders assembled keeping a minimum gap of 30 mm between them.
  7. Imported high-pressure cylinders are assembled inside a sturdy frame in three independent banks – Low bank, medium bank & high bank.
  8. The frame having a lifting eye to facilitate lifting with the crane or forklift with the full weight of cylinder filled with gas and have suitable tie-down clamps at the base.
  9. The cascade painted with special paint to withstand environmental conditions. All fitting and valve are of SS 316 rated for the working pressure. All tubing is of SS 316 conforming to ASTM A2269 suitable to absorb contraction, expansion, and vibration.
  10. The pressure gauge is also mounted on each bank to check the pressure of the gas inside the cylinder.

CNG cylinders:

  1. The natural gas storage cylinders have various designs. Design types include type 1 which is all metal; type 2 which has metal liner & hoop wrapped composite liner reinforcement; type 3 has metal liner & full wrapped composite liner reinforcement; type 4 has non-metallic liner & a full wrapped composite reinforcement.
  2. Metal containers & liners are typically steel or aluminum. Reinforcement liners are typically carbon or glass fibres in a resin matrix.
  3. CNG Cylinders are designed for a specified temperature, essentially a specified density of fuel. Fueling stations normally fill the cylinder up to 125% of nominal service pressure to avoid under charging but this highly depends on ambient temperature.

Cascade filling

  1. The storage cylinders are divided into high, medium & low bank arrangement for fast filling & maintaining high pressure in vehicle on-board cylinder.
  2. The vehicle on-board cylinder is connected to low pressure bank & gas is transferred from low bank to vehicle cylinder.
  3. As the pressure in low bank falls, the pressure in on-board cylinder increases & gas flow decreases. When the flow-rate decreases to a pre-set level, the system switches to medium bank . When the pressure in medium bank also falls, the flow rate decreases & when it decreases to a pre-set level, the system switches to high bank to complete the fill.
  4. When all the three banks of the cascade are empty, the compressor is automatically turned-on to refill the banks.
  5. The compressor fills the high pressure bank first & switches to medium & low banks. This assure that high pressure bank is kept at maximum pressure all the time.
  6. Priority panels are generally installed for determining the sequencing & priority of flow of CNG from the compressor to the cascade banks.

Priority Panel

  1. Priority panels are installed for determining the sequencing & priority of flow of CNG from the compressor to the cascade banks.
  2. The priority panel directs the gas flow from the compressor to the high bank, medium bank, or low bank of the storage cascade, depending on demand.
  3. If the storage pressure is low, the discharge gas is sent directly to the vehicle to provide faster fueling. Priority panel holds the responsibility to supply the gas from the compressor unit to the dispenser, stationary cascade, and mobile cascade logically. Each line here are provided with non-return valves & pressure transmitter.
  4. The stationary cascade is having the following cylinder banks: High bank, Medium bank, Low bank
  5. The main objective of providing high bank, medium bank and low bank facility is to meet the customers with irrespective of pressure gauge.
  6. When the stationary cascades are filled, compressor will be stopped promptly. Since, frequent operation of the reciprocating compressor leads to wear and tear. Setting up of a stationary cascade bank is the excellent alternative to escape away from wear. Thus, by doing this we can intensify the compressor life.

A typical priority panel works in the following procedure;

  1. When no vehicles in the refueling stand, natural gas is being passed to a high bank, which is getting the number one priority but next to the vehicle filling. Since any vehicle coming in the middle it has to fill the gas to the vehicle first after that to high bank.
  2. When the high bank is filled, then the priority goes to the medium bank.
  3. When the medium bank gets filled, it goes to the lower bank.
  4. When all the banks are filled, gas will be diverted into the light commercial vehicle is called as LCV in general.
  5. At the time of high bank or a low bank are filled, any vehicle comes to refill the gas, Priority panel cuts the gas from filling any banks and will send to the vehicle. The order or vehicle filling from the priority panel is having different sequences. First, the gas is filled from the low bank into the vehicle cylinder, on near equalization of pressure, , the medium bank starts filling the cylinder, and again at the equalization of pressure, HP bank starts filling the vehicle cylinder till pressure equalizes.
  6. The priority panel generally has the following set points – High bank (220 bar), Medium bank (210 bar) & low bank (200 bar).

Technical Specification of Cascade:

  1. Each CNG cascade shall have a storage capacity of a minimum 2000 liters water capacity at 15 degrees Celsius.
  2. The number of cylinders in the cascade shall be divided into independent banks of the low, medium, and high-pressure banks for differential storage pressure as required by 3 lines sequential dispensing system of the electronic CNG dispensers.
  3. The vendor shall optimize the number of cylinders in each bank for maximizing the recovery from the cascade storage.
  4. All cylinders are to be painted white with signal red on the cylinder neck portion as per directions of Chief Controller of Explosives (CCOE), Govt. of India, and assembled into a freestanding, galvanized steel welded frame. In the case of a painted frame, the paint shall be chosen, primed, and applied to have a service life of 10 years. The exterior surface is required to be corrosion-free for ten years and to have fade-free life without oxidation of paint surface for five years in an environment of bright sunlight with intense UV content.
  5. Cascade frame shall have provision for lifting by crane from the top of the frame with the full weight of cylinders filled with compressed natural gas (at 255 bar pressure) and shall have tie-down clamps at the top and bottom.
  6. The bottom and top of the frame shall be reinforced to prevent any twisting or stress to cylinder connection during lifting by crane. Cascade frame shall be provided with an inbuilt canopy to protect the cylinders from rain and sun.
  7. All cylinders shall be mounted inside the frame horizontally. Proper guards shall be provided in the cascade frame, for protecting the cylinder valves and other fittings against any damage.
  8. Cascade storage capacity – The water storage capacity of each cascade shall be 2000 liters at 15 degrees Celsius with no negative tolerance.
  9. The overall dimension of cascade & weight with gas shall not exceed as given under Height: 1.45 Mtr. , Width: 1.9 Mtr. , Length : 2.7 Mtr. & Weight: approx. 4.0 tons.
  1. Inbuilt canopy shall be provided with the cascade for protection from sun and rain. The canopy shall be in line with the approved drawing of CCOE and shall be designed to take care of the wind load, thrust, vibration, etc. The canopy shall be designed in such a fashion that it covers the complete cascade system. The material for inbuilt canopy shall be GI of minimum 16-gauge thickness. The canopy shall be designed to provide adequate ventilation/dispersion of gas in the event of any leakage. Due to space constraints, we may install the stationary cascade on the top of the canopy of the compressor. Vender shall consider the same during designing.
  2. The frame structure shall not allow lateral and rotational movement of cylinders during regular road transport under any circumstances. The vendor shall take into account rough patches or bump on roads.
  3. The frame structure of each cascade shall be capable of withstanding impact up to 4-G (four times gravity) from any direction without any distortion.
  4. All items used in the frame shall be weather-proof.
  5. Each cascade shall be provided with an identification plate of stainless steel material and of adequate size. The identification plate shall carry the name and logo of the entity, including cascade serial no., name of the manufacturer, year of supply, cylinder serial nos. last hydro test date, maximum working pressure, a total water capacity of the cascade, next hydro test due date, and entity serial no. (to be left blank) The matter on the identification plate shall be of suitable size and shall be engraved.
  6. Each bank shall have one 4” liquid filled pressure gauge (0 – 400 kg/cm2g) connected in the piping with a 3-way isolating valve. Thus each cascade shall have three pressure gauges. Pressure gauges shall be securely mounted and casing shall be of Stainless Steel material. The pressure gauge shall have shutter proof glass.
  7. Construction, licensing & inspection, markings, fitment, handling, storage, and all other relevant aspects of cylinders and fittings shall be governed under ‘The Gas Cylinder Rules, 2004’ and shall be approved by Chief Controller of Explosives (CCOE), Government of India. In case any factor, parameter or requirement given in the tender or offered by the vendor is changed or modified by CCOE or any other Indian Statutory Authority before delivery of cascades to the site(s), same shall be carried out without any extra price implication to the entity and to the satisfaction of CCOE or statutory authority, as the case may be.
  8. A minimum gap of 30 mm between cylinder to cylinder shall be provided and maintained.2
  1. Each cylinder shall be equipped with a high flow shut-off valve and safety relief device consisting of a fusible burst disc assembly. The cylinder valve shall conform to requirements of IS: 3224 or as per the design approved by Chief Controller of Explosives (CCOE), Govt. of India. The burst disc shall rupture on excess pressure as well as excess temperature either individually or combined, as per internationally accepted code of practices. The orifice of the cylinder valve shall be designed for high flow to permit the combined flow of 100 kg/min from each bank at a pressure of 250 bar.
  2. The burst disc discharge shall be manifolded to a common header for safe venting. All tubing & fittings used for vent Mani folding shall be SS316.
  3. All cylinder valve fittings and pipe fittings shall be rated for the full range of temperatures and pressures and the valve/fittings shall show stamp or otherwise permanent mark on the body to indicate the service ratings.
  4. Inlet connection (for filling) to cascade and outlet connection (for discharge) from cascade: Full bore ball valves for isolation shall be provided on each inlet /outlet connection along with separate bleed valve. End terminations for connection to dispenser tubing shall be minimum ¾” OD tube fitting.
  5. Piping/Tubing: The cascade cylinders shall be manifolded in 3-bank configuration and all inter-connection tubing (including tubing between the main header to individual cylinders) of minimum 3/4″ O.D. will be sized according to requirements of code ANSI B31.3, looped for contraction, expansion and stress relief. The sizing of connecting tubing between each outlet and its associated cylinders shall be such that the total incoming flow areas shall not be less than an outgoing area. Piping/tubing shall be suitably clamped. Materials used for the piping shall be stainless steel 316 – high quality fully annealed seamless conforming to ASTM A269 with maximum hardness Rb80 or less and suitable for bending and flaring. O.D tolerance shall not exceed +/-0.005”.
  6. Fitting: All fittings including valves shall be of material SS316 conforming to ASTM A269. Cascade isolating valves shall be full flow ball valves.
  7. Vendor to design the flow rate of the whole cascade in such a way that gas speeds are kept low. The tube, fittings, and valves shall be designed for high flow to permit the combined flow of 100 kg/min from each bank at a pressure of 250 bar.

Cylinder Specification:

  1. Design code: As per Gas Cylinder Rules, 2004 and IS 7285-1988 and prior approval of Chief Controller of Explosives (CCOE), Government of India.
  2. Water storage capacity – Minimum 75 liters per cylinder
  3. Working pressure – 255 kg/cm2g
  4. Neck threads – Screwed 1” dia standard. The valve shall be attached to the cylinder neck by screwing only. The cylinders shall be fitted with appropriate valves conforming to the requirement of IS-3224 or as per the design approved by CCOE, Govt. of India.
  5. Collar thread – Dia 80 X 11 T.P.I
  6. Material – Seamless chrome-moly alloy steel.
  7. Painting of cylinders – Cylinders shall be painted as per Gas Cylinder Rules/ IS code as follows: White color on the cylinder body, Signal red on cylinder neck portion & The cascade frame shall be painted yellow. The first coat shall be applied with epoxy primer to achieve DFT of 25 microns. This will be followed by the second and third coat of epoxy paint and the DFT per coat shall be 25 microns. The paint shall be chosen, primed, and applied to have a service life of 10 years. The exterior surface is required to be corrosion-free for ten years and to have fade-free life without oxidation of paint surface for five years in an environment of bright sunlight with intense UV content.
  1. Cylinder Test Certificates – Vendor to obtain CCOE approval/license to fill and store CNG in cylinders manufactured by submitting original production certificates for each cylinder. All cylinders shall be subjected to hydraulic stretch tests at a minimum pressure of 422 bar or to pressure as determined and approved by CCOE and as required under rules 39 to 42 of Gas Cylinder Rules, 2004 (India). Test certificates shall also be submitted to the Chief Controller of Explosives for physical properties and chemical analysis of the steel used in the manufacture of cylinders (batch-wise) along with mill test certificates. All certificates shall be submitted to CCOE in original duly signed by approved inspecting authority.
  2. Inspection and testing: Vendor shall carryout all standard shop tests/inspection for cylinder manufacture as per vendor’s recommended and CCOE approval QA/QC procedure. A copy of the tests/inspection procedure shall be provided to the entity. Each cascade assembly with all tubing, fittings, valves shall be pressure tested with air at the required pressure to ensure no leakage prior to dispatch, and the same will witnessed by the entity or its authorized representatives. Cascade assembly shall be purged with N2 prior to dispatch.
  3. Field Performance Test: Once cascades have been received and connected to our sites, the vendor shall depute his representative to demonstrate satisfactory operability of the system within specified parameters for a period of two (2) days after commissioning. There will not be any backflow between any bank with all valves open during actual operation. If any leakage, backflow, or any other malfunction is observed or noticed on any cascade during the test period, the vendor shall do the necessary repairs and repeat the test afresh and without any extra cost to the entity.

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