Preventi­ng the water hammer in high rise building - Final report

Hong Kong Institute of Vocational Education

(Tuen Mun)

Engineering Discipline

Department of

Real Estate and Facilities Management

Higher Diploma in Building Services Engineering

Programme Code: EG114701

Preventing the water hammer in high rise building

Industry-Based Student Project

Final year report

Student Name/No. : Kam Kuen/ 140670080

Supervisor: Ir Kin Fu

May 2015

“I declare that this report is my own work and was not copied from or written in collaboration with any other person.”

Signed: _____

Content

Introduction P.4

Background P.5

Objective P.6

1. Cause and prevention method in high rise building P.7-28

1.1 Cause 1: Rapid closure and opening of valve P.7-17

1.1.1 Installing water hammer arrestor in high rise building

1.1.2 Important location of water hammer arrestor in high rise building

1.1.3 Calculation of the water hammer arrester size in the pump room

1.1.4 Case study of the portable water pump room

1.2 Cause 2: Pump startup P.17-21

1.2.1 Solution: replacing the direct on line starting by the variable frequency drive

1.2.2 Working principle of the variable frequency drive

1.2.3 Comparison between direct on line starting and variable frequency drive

1.2.4 Summary of the variable frequency drives

1.3 Cause 3: Pump backflow P.21-25

1.3.1 Solution: installing check valve

1.3.2 Type of silent check valve

1.3.3 What is the ideal check valve?

1.4 Cause 4: Air trapped in the pipeline P.25-30

1.4.1 Source of air

1.4.2 Impact of air on high rise building

1.4.3 Solution: installing automatic air vent

1.4.4 The installation of automatic air vent

1.4.5 Solution: applying the pressure release method

1.4.6 The process of the pressure release method

Content

2. Other prevention method P.30-46

2.1 Reduce water pressure in the high rise building P.30-43

2.1.1 Application of PRV in the portable water supply system

2.1.2 Method of Operation in the portable water system

2.1.3 Installation Guidelines in the portable water system

2.1.4 Application of PRV in the flushing water supply system

2.1.5 Method of operation of pressure reducing valve

2.1.6 Installation guidelines in the flushing water supply

2.1.7 Case study: Calculation for the location of PRV

2.2 Proper pipe system design P.43-45

2.2.1 Good and adequate pipe support

2.2.2 Selection of the material of the pipe support

2.2.3 Minimize the turning point in the piping system

2.3 Water hammer software for detecting the water hammer P.45-47

2.3.1 Software with complex modelling capabilities

2.3.2 Software with limited modelling capabilities

2.3.3 Interpretation of results

3. Learning outcome P.47-54

4. Degree of Achievement P.55

5. Conclusion P.55

6. Reference P.56

Introduction

Water hammer, also known as hydraulic shock, is a sudden buildup of pressure in a pipe due to change in direction or speed of the water inside, which can cause the pipe to burst and damage equipment.

The most common cause of water hammer is a valve closing quickly, momentarily stopping the flow of water through the pipe and increasing pressure in the upstream side and decreasing pressure in the downstream side of the valve.

Other causes of water hammer include a rapid pump startup or shutdown and air pockets in pipes that create a similar vacuum effect that leads to pipes imploding.

Therefore, in summary, there are the three most common causes of the water hammer: rapid closure and opening of valve, pump application challenge of startup and shutdown and air trapped in the pipeline.

When there is sudden change of the liquid velocity in the pipeline, a metallic and repetitive 'bang, bang, bang', or even sometimes a violent 'boom' accompanied by vibration may be heard. Most residents in high rise building will probably have experienced one of these at some time. When water hammer occurs, a momentary abrupt pressure change of over 10 MPa may occur inside the piping.

This impact can severely jar piping, equipment or machinery housing, possibly resulting in damage not only to gaskets in junctions, but also to valve flanges .....[read full text]

We have find the value of P2, V2, P3, V3 so we can calculate the work done per 100cm3 that used to convert the kinetic energy produced by water hammer by using the equation:

Step 5: Using ratio method to find out the volume required for convert the kinetic energy into the work done during compression.

Until now, we can use the relationship between the work done and kinetic energy to find the designed volume (V1):

We can use the ratio method to find the actual volume of air in the maximum compression stage (P3).

For example, the work done per 100cm3 air cushion during compression can convert 60J kinetic energy, and then if there is 600J kinetic energy produced by the water hammer, we should set the size of air cushion in water hammer arrestor is 600cm3 to eliminate the water hammer problem.

Finally, we can apply the Boyle’s law: P1V1=P3V3=C to find the actual size of the arrestor (P1).

1.1.4 Case study of the portable water pump room in the Tin Hau construction site (THE PAVILIA HILL Tower 2)

The information about the potable water pump room

Step 1: Calculate the pressure rise (water hammer) by using the joukowsky equation

Step 2: Calculate the kinetic energy during the pump start

Step 3: Find the pressure of pre-charge state, charge state, maximum compression state of the air cushion of the arrestor

Step 4: Find the work done during compression of the air cushion per 100cm3 by using the Boyle’s law.

Step 5: Using ratio method to find out the volume required for convert the kinetic energy into the work done during compression.

1.2 Cause 2: Pump startup

Direct on line starting is the common method to start the pump motor. The working principle is very simple. It is closing a contactor and applying full voltage to the motor for operating. However, there is a large drawback in the direct on line starting.

When full-line voltage is applied to start a motor that is directly coupled to the shaft, the pump is accelerated from zero to full speed quickly, often in less than ¼ second.

In the case of the pump, the excessive accelerating torque produced by starting the motor direct online can cause the pump to come up to speed quickly, potentially causing surges or hammering.

1.2.1 Solution: replacing the direct on line starting by the variable frequency drive

Podium irrigation water booster pump control panel

VFD inside the control panel

As the government suggest that system pressurization and de-pressurization rate should not exceed 0.25 bar per second or full system pressurization within 30 seconds whichever yields the higher period of time.

To achieve this, all pumps should have a controlled slow starting and stopping mechanism to eliminate sudden pressure surges during start-up and shut down.

As a minimum this should consist of a slow start pump motor controller with at least 15 seconds of ramp up and ramp down time.

Better still is a variable frequency drive (VFD) which gives even greater flexibility in ramp-up and ramp down times and also has the advantage of being able to match the duty point of the pump precisely to the process requirement.

1.2.2 Working principle of the variable frequency drive

The operation of the drive is as follows:

Power first goes into the rectifier, where the 3-phase AC is converted into a rippling DC voltage. The intermediate circuit then smooth and holds the DC Voltage at a constant level or energy source for the inverter. The last section, the inverter, uses the DC voltage to pulse the motor with varying levels of voltage and current depending upon the control circuit.

To reduce the sudden change in liquid velocity, the control section uses the frequency, voltage and phase angle to control the inverter. Finally, the inverter can control the acceleration and deceleration of the pump to prevent the water hammer.

1.2.3 Comparison between direct on line starting and variable frequency drive

Starting flow characteristic:

This chart shows the differences flow characteristics between across direct on line and the pump control soft starter during startup. Direct on line can rapidly increase the flow rate while soft starter follows a smooth s-curve for an extended time. This mean VSD can reduce the sudden change in flow by acceleration part at the pump water and extending the time to produce a hundred percent flow.

There are no sudden changes in part and hence there is a reduction in sudden change in velocity

Therefore water hammer is reduced through the piping system

Stopping flow characteristics:

This chart shows the differences flow characteristics between across direct on line and the pump control VFD during startup

Direct on line starting reduce the flow very rapidly while pump control refuse a sudden change in flow by controlling the deceleration of the pump water and extending the time to get the stable flow and prevent the valve slamming.

Water hammer is reduced in the piping system.

1.2.4 Summary of the variable frequency drives

Variable frequency drives can eliminates water hammer and check valve slam in plumbing systems by extending stopping time and provide s-shaped torque curve to ramp and ramp down the flow.

Case study

1.3 Cause 3: pump backflow

Backflow is the phenomenon in the high rise building for an unwanted flow of water in the reverse direction when the pump shutdown.

The reverse water will collide with the water pumped by the startup. In the point of the collision, sudden change of pressure will be occur. Finally, it cause the high pressure water hammer.

1.3.1 Solution: installing silent check valve

Backflow problems can be eliminated or greatly minimized by installing a spring assisted SILENT check valve.

Check valves do not rely on gravity or fluid flow for their closure. However, when the forward velocity of the fluid slows, the total force of the forward water flow is less than the tension of the spring. Therefore, spring assist on the valve starts to close the disc.

Due to the spring assist and the relatively short distance the disc must travel, by the time the forward velocity has decreased to zero, the valve disc has reached the seat and the valve is closed.

As the valve closes, the reverse flow will be stopped by the closure of the check valve, which cannot colli.....

To solve the problem, we can use the pressure release method and install automatic air vent to be against the water hammer.

1.4.3 Solution: installing automatic air vent

When liquid is pumped through the system at startup, the initial air inside the piping is pushed into the air vent by the pressure of the flow. The float remains in the lower part of the air vent, allowing the valve to remain open and continuously discharge air.

After the initial air venting, liquid flows into the air vent. The float rises with the rising liquid and closes the valve.

If air enters the air vent while it is closed, the liquid level drops and the float lowers, allowing the valve to open and discharge air once more.

1.4.4 The installation of automatic air vent

There are some installation rules of automatic air vent:

1. In the point where flow turns downward, it takes mighty flow to move air downward. Some of the air may trap into the lower turning points

2. The location of the air vent should be the points where flow turns downward to greatly reduce the chance of water hammer

3. Besides, the air vents should be installed at the system high point because air has lower density than water. It will move upwards if there are no barrier.

4. The air vents must be install vertically rather than installing horizontally

1.4.5 Solution: applying the pressure release method

Many high rise buildings would be placed with vent pipes at the system high points to manually bleed off the accumulated air.

The method is simple and low-cost. However, it has proved impossible to predict the time to bleed the air.

As the pipeline of the building must be with air because of the pump startup, This means we should regularly use the method to bleed the air which leads to time wasting.

1.4.6 The process of the pressure release method

Step 1: open the gate valve slowly and allow air bleeding from the vent pipe

Step 2: Close the vent pipe when the water flow become stable

1.5 other prevention method

1.5.1 Minimize the turning point in the piping system

2. Other prevention method

2.1 Reduce water pressure in the high rise building

Reducing the water pressure can effectively buffer the effect of water hammer. Pressure reducing valve is one of the method to reduce the pressure to a desired pressure in the high rise building.

During my internship in the Majesty Company, there are two systems: Flushing water system and Portable water system required to be installed PRV for preventing the water hammer

2.1.1 Application of PRV in the por.....

The inlet pressure has no influence in either opening or closing of the valve. Because of this, inlet pressure fluctuation does not influence the outlet pressure, thus providing inlet pressure balancing.

The size of PRV depend on the size of the connection. Take an example in my company. If the size of the pipeline is 80mm, we would choose the PRV with connection size 80mm. After the selection of PRV, we can find the relevant data from the table such as the dimensions, weight and kvs-value etc

Besides, in the installation example, we find the minimum distance from wall to centre line of pipework from the different size of PRV. For instance, the PRV with 80mm connection should be installed 130mm apart from the wall.

This is my observation in the PRV installation system for the portable water water supply system in the construction site.

2.1.3 Installation Guidelines in the portable water system

• Install in horizontal pipework with spring bonnet directed upwards

• Install shutoff valves

• The installation location should be protected against frost and be easily accessible

-Pressure gauge can be read off easily

-Simplified maintenance and cleaning

• Install downstream of the filter or strainer

-This position ensures optimum protection for the pressure reducing valve against dirt

• Provide a straight section of pipework of at least five times the nominal valve size after th.....

The inlet pressure has no influence in either opening or closing of the valve. Because of this, inlet pressure fluctuation does not influence the outlet pressure, thus providing inlet pressure balancing.

The size of this type of PRV depend on the size of the connection. Take an example in my company. If the size of the pipeline is 40mm, we would choose the PRV with connection size 40mm. After the selection of PRV, we can find the relevant data from the table such as the dimensions, weight and kvs-value etc.

Besides, in the installation example, we find the minimum distance from wall to centre line of pipework from the different size of PRV. For instance, the PRV with 40mm connection should be installed 70mm apart from the wall.

This is my observation in the PRV installation system for the flushing water supply system in the construction site.

2.1.6 Installation guidelines in the flushing water supply

• Install in horizontal pipework with filter bowl downwards.

• Install shutoffs valves

• The device downstream should be protected against frost and be easily accessible:

-Pressure gauge can be read off easily

-With clear filter bowl, degree of contamination can be easily seen

-Simplified maintenance and cleaning

• For residential applications where maximum protection against dirt is required, install a fine filter upstream of the .....

The equation of the maximum pressure set by us:

Maximum water pressure suggested by the architect X 5/6 = Safety water pressure

Therefore, the safety pressure would be 60M X 5/6 = 50M

Before starting the calculation of the pressure head in all the residental floor, we should know the two source of the increase of the pressure head:

One source is due to the startup of the booster pumps in upper roof, Usually, we can find the pressure head of the booster pumps from the catalogue sent by the supplier.

As you see the booster pump catalogue, the pump would produce 40M head. However, the actual pressure head would not be 40M because of the three source of pressure head: static head, pipe fiction loss and head loss of fittings. The calculation would be done by the supplier and the calculation steps shown below:

After calculation of the head loss, we can find the head loss of the system and then we find the actual pressure head produced by the booster pump by subtracting the designed pump head with the head loss of the system. Finally, we can find that the head loss of the booster pump would be around 25m

Another source is the height of each residential floor. The height of each residential floor is equal to the pressure head. When the water flow from the above to the below, the level difference between the above and the below would convert to the pressure head due to the gravity. Therefore, the water pressure of each of the residential floors would be found by adding the pressure head due to booster pump and the level difference.