How to Design WATER SPRAY SYSTEM FOR HV TRANSFORMER?

System Description

High Velocity Water Spray Systems (HVWS):

HVWS, is generally used in systems, having high flash points. The velocity of the water sprays, hitting the system under fire is critical for successful fire extinguishing action.

High Velocity Water Spray Systems Philosophy

HVWS shall be designed as per NFPA 15 regulations. HVWS shall consists of above group piping, along with relevant fittings, deluge valves, isolation gate valves, spray nozzles, Quartzite bulb detector and pressure switches. HVWS system shall be equipped with the provision of automatically detect, control & extinguish any outburst of fire. The system shall allow hydraulically open the deluge valve thus allowing water to be sprayed on the equipment/area through projector nozzles in
the form of a solid conical emulsifying spray.

Isolation gate valve shall be provided on upstream and downstream side of the deluge valve. Fast acting butterfly valve shall be provided as a by- pass to deluge valve, so that this valve can be kept closed and can be operated manually in case of malfunction of deluge valve. The pressure at the hydraulically most remote point in the network shall not be less than 3.5 bars for outdoor transformers as per NFPA 15.

Plant Areas/ Equipment’s covered under HVWS are:

1. Turbine Lube oil / Dirty Oil storage tanks
2. All oil filled Transformers (Oil capacity more than 2000ltr.)
3. All type of oil storage tanks.
4. Boiler’s burner and its surroundings.

Design Basis:

The system shall be designed completely as per NFPA 15 and other
international standards

Application Rate: 10.2 lpm/ m2 (Minimum) of the surface area of the entire transformer including the bottom surface, radiators, conservators etc.

A minimum pressure of 3.5 bars shall be achieved at the hydraulically remotes sprayer. However, pressure at the hydraulically favourable sprayer shall not exceed 5 bars. The velocity in the feed pipes shall not exceed 10M/second.

The water spray system shall be provided with deluge valve assembly actuated by QBD (Quartzoid Bulb Detector) on water line (wet system). In addition to auto actuation by QBD, facility of manual actuation of deluge valve, locally through a push button deluge valve panel and hand lever shall be provided.

Spray Nozzle (Projectors) Selection/QTY Calculation:

Placing of spray nozzles shall be such that their spray nozzles should cones overlap each other.

Field obstruction if any affecting the spray pattern of the nozzle must also be considered.

K-Factor shall be calculated with respect to protected equipment area, (theoretical/tentative) nozzle QTY and design density/rate of application.

Spray angle, Spray Pattern & discharge characteristics graph of manufacturer data sheet shall be referred while placing/calculating the spray nozzles.

According protected equipment area, K-Factor, Selected Nozzles Spray Angle, (theoretical/tentative) nozzle QTY and design density/rate of application, theoretical water requirement shall be calculated.

Accordingly final spray nozzle QTY shall be calculated with respect to theoretical water demand.

With reference of theoretical water demand deluge valve size shall be selected according available flow range & size of deluge valve as per approved manufacturer standard.

Spray Piping:

Transformers/Equipment’s/Tanks shall be protected using rings of nozzles. Projectors on the rings shall be located at not less than 500mm and not more than 800mm from the Transformer/Equipment surface. The horizontal and vertical distances between the projectors shall be maintained in such a way that their spray patterns intersect on the surface of the Transformer/Equipment.

HVWS System operation philosophy:

Operation of HVWS system:

The system is auto actuated wet pilot based high velocity (HV) water spray system. The system consists of detection line and protection line around the transformer boundary. The QBD detectors are designed to operate at 79 deg Cel. In fire condition, When the rated temperature is reached the bulb shatters and water flows through the sprinkler/QBD and due to low pressure at deluge valve upstream cause to deluge valve opening action, will start automatic sprinkler protection line of sprayers. The system can also be started by Push Lever (Emergency valve) on the Deluge Valve Trim. The bypass line is provided on this spray system, so that the spray system can be manually operated without any automation, in case need arises. In addition to above Deluge Valve Panel has been considered to monitor as well as operate the HVWS system.

Deluge Valve panel shall be provided with following IP/OP signals/notifications.

Water Demand Calculations for Transformer Protection

Step: 1 : Determine Preliminary information about the Transformer such as the following. Transformers come in many sizes and configurations. Before attempting to design the protection, it is essential to have the following information:

• Length
• Width
• Height of transformer
• Location and height of bushings
• Height and location of lightning, if any
• Size and location of oil expansion tank, if any
• Location of any switch boxes and any equipment that may interfere with water
  distribution
• Size of transformer, i.e., high and low voltage
• Phase of transformer, either single or three phase
• Direction of incoming high voltage and low voltage wire or bus bars to the
  transformer
• Setting of transformer, whether surrounded by concrete or crushed rock
• Elevation of bottom of transformer above grade
• Location of radiators and distance between radiators. When space between
  radiators exceeds 12 in (.3 m)
• it must be covered
• Size and location of, if any
• Estimate of possible effects of wind, and size and location of any wind
  protection.

Design Notes :

1. If the transformer is not existing, it will be necessary to obtain a manufacturer’s dimensional drawing of the proposed transformer.
2. The drawings for the transformer should be made to a large scale, e.g., 3/8” to 1’-0” or 1/2” to 1’-0” (1/30 or 1/25), and there should be three views: top, side and bottom. If more than one ring is necessary an additional plan view may
   be necessary.
3. In addition to the transformer, a detailed plan should be drawn to show the general view, such as firewalls between transformers, water supply location, valve location, electrical poles, and any other obstructions that may interfere
   with the sprinkler piping.
4. Transformers present particular design problems for water spray protection, primarily because of their irregular shape and the necessary clearances to be provided from high voltage wiring. Generally speaking, there is much more interference with the water flow on the surface of the transformer than there is on a tank. For this reason protection systems for transformers generally involve a large number of small capacity nozzles. Often it will be necessary to put more water on the transformer than is actually required simply to achieve coverage. It is most useful to use a large-scale drawing of the transformer and project theoretical nozzle discharge patterns on it to get an idea of the type of coverage to be expected.
5. Transformers are generally protected using rings of nozzles around the transformer with the top ring being located near the top of the transformer and subsequent rings being located every 12 ft (3.6 m) from the top to the bottom of the transformer or beneath each continuous obstruction.
6. Nozzles are also employed to spray water on the bottom of the transformer in the event it is more than 12 in (.3 m) above the ground. In addition, if the ground is covered with solid material such as concrete or asphalt, nozzles must be located to wash fl flammable liquid away from the transformer. Nozzles must be located so as to spray the proper amount of water into the “design area”.

7. To determine the various design areas of the transformer, consider that the elements of the transformer are a collection of simple geometric fi gures (cylinders, cubes, etc.). Make a plan and elevation view of the simplify ed transformer concept.
8. If the transformer is located 12 in (.3 m) or more above grade, also make a bottom view. Neglect small protrusions or increase size of fi gure slightly to compensate. Radiators should be considered as a single unit unless there is more than a 12 in (.3 m) space between them. In this case, they must be considered as multiple units.

Required Density = d
Required Grade Density = dg

Step: 2: Determine the design area for the top and sides of the transformer.

Step: 3: Determine the design area for the radiator Body

Step: 4: Determine the design area for the oil Tank

Step: 5: Determine the design area for the bottom (only if the transformer is 12 inches for more above the grade)

Step: 6: Determine the design area for the grade (The design area for the grade is the area that appears on the simplified bottom view of the transformer plus an area extending 3 ft (.9 m) on all sides of the view. Grade protection is required only when a non-absorbing surface such as concrete or asphalt paving is employed. Grade surfaces such as gravel or crushed rock do not normally require nozzle protection. Grade protection is not required directly under the transformer unless it is
located at least 12 in (.3 m) above grade.)

Step: 7: Determine the water requirement for Top and Sides

Step: 8: Determine the water requirement for Bottom

Step: 9: Determine the water requirement for Grade

Step: 10: Determine the water requirement for Entire Transformer.

Step: 11 : Assume Nozzle Pressure ( Between 3.5 to 10.5 Bar)

Step: 12 : Select the Probable Nozzle Arrangement

Follow below given guidelines

a. Minimum electrical clearances
One of the most important considerations in locating the piping around the transformer is the distance of the pipe from the electrical components or energised parts, such as bare cables, bus ducts, and the low voltage and high voltage bushings. The clearance between any portion of the water spray equipment and the unenclosed or uninsulated electrical components, at other than ground potential, should not be less than given in the following table. These clearances are for the altitude of 3,300 ft (1,000 m) or less. The distance should be increased at the rate of one percent for each 330 ft (100 m) increase of altitude above 3,300 ft (1,000 m).

b. Distance of Nozzle from the surface.

Unless the transformers are located indoors where there are no wind conditions, the surface of the nozzle should be located no more than 2 ft (.6 m) from the vertical surface to the transformer

c. Coverage for Transformer Top

Generally, 30, 60 or 90-degree spray nozzles are installed in this top loop with the nozzles located approximately 1 to 2 ft (.3 to .6m) above the transformer top and pointed so that the water will impinge upon the transformer. Water should not be directed at the high voltage bushings. The above nozzles have a maximum effective horizontal throw of 6 ft at 30 psi (1.8 m at 2.0 bar). It may be desirable to locate nozzles at the corner to achieve increased coverage.

d. Horizontal distance between nozzles

The horizontal distance between nozzles should be such that their patterns intersect along the horizontal line. For nozzles located 2 ft (.6 m) from the surface, the following horizontal distances should be used

e. Two sets of nozzles from the same ring

Because, generally speaking, small capacity nozzles will be used, it will often be possible to extend the nozzles above and below the loop by means of a nipple. Nipples longer than 2 ft (.6 m) generally require additional support.

f. Bottom nozzles

If the transformer or radiator is located more than 12 in (.3 m) off the ground, it is necessary to protect the bottom. This is generally done with wide angle spray nozzles pointing upward.

g. Between radiator protection

If the radiators are more than 12 in (.3 m) apart, nozzles must be arranged to spray into this space. A nozzle angle should be selected so that the cone diameter at the entrance is equal to or slightly larger than the space between the radiators.

h. Rundown considerations

Rundown will occur on smooth, vertical surfaces. Projections from the surfaces, however, will “roof off” certain areas which would normally be covered by rundown. These “roofed off” areas usually require specifi c nozzle coverage.

I. Vertical distance between nozzles

For unobstructed vertical surfaces with no “roofed off” area and unobstructed rundown, a maximum vertical distance between nozzles is 12 feet (3.6m). In practice, however, unobstructed areas of this size are rarely encountered.

j. Nozzle direction

Nozzles protecting the transformer top should be aimed slightly down so that all of the water impinges upon the transformer with either all on the top or some on top and some on vertical sides. Nozzles protecting vertical sides and bottom should
point directly at the surface to be protected. Nozzles covering irregular areas should be located for best coverage generally spraying into corners. Overspray must be avoided. Nozzles covering space between radiators should be arranged to spray directly into the open space.

k. Overspray

If nozzles are located too far from the surface, or if the angle is too large, there will be overspray, that is, water coming from the nozzle will not impinge on the transformer and will be wasted. In order to avoid overspray, locate nozzle closer to
the transformer or use a smaller angle. Always observe electrical clearances.

l. Grade protection

If the transformer is located 12 in (.3 m) or more above a non-absorbing surface such as asphalt or concrete, nozzles must be located under the transformer pointing down and outside the transformer covering an area 3 ft (.9 m) around the
transformer pointing generally outward. The purpose of the grade protection nozzles is to wash flammable liquid away from the transformer (consider grade slope). It is often possible to feed both bottom protection nozzles and grade protection nozzles from the same pipe. In some cases (usually small transformers) it may be acceptable to use an open pendent sprinkler in the upright position. No grade protection is required when readily absorbs the flammable liquid.

m. Wind

Often, because of transformer configuration and electrical clearances, it will not be possible to locate spray nozzles close to the areas that they are expected to protect. When the installation is outside, the effect of the wind must be seriously
considered. Small spray nozzles operating at high pressure produce small drops that are particularly susceptible to being blown away by the wind. It may be necessary to increase water density in questionable conditions.

Step: 13: Determine the number of nozzles based upon above arrangement

Step: 14 : Determine K Factor of the nozzle based upon flow and the pressure

Step: 15: Select Appropriate K Factor from Manufacturer catalogue.

Step: 16 : Find the total amount of water delivered into each design area by the probable nozzle arrangement

Step: 17 : Find the total amount of water delivered into each design area by the probable nozzle arrangement
(Compare the water delivered to the design area by the probable nozzle arrangement with that required. Increase or reduce nozzle sizes and pressures as necessary. (Note that because of the irregularities of transformers, many nozzles are required to provide coverage. In addition, because of electrical clearances, some of these nozzles are required to throw at maximum distances. For this reason it may not be possible to reduce the number of nozzles or the operating pressure far enough to approach the minimum. In practice it may be possible to reduce the required pressure below 30 psi (2 bar); however, this should not be done in the
design stage.)

Step: 18 : Perform Hydraulic Calculation & determine pipe Sizing.