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Hong Kong Institude of Vacational Education,hong kong

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tuen mun,Ir Chow Kin Fu,2016

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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. Conclus.....[read full text]


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When the valve is closed or opened very quickly, the valve will acts as barrier to stop the water flow.

This will cause the sudden change in both velocity and pressure. Finally, it will produce water hammer and make damage to pipeline system.


1.1.1 Installing water hammer arrestor in high rise building

Installing the water hammer arrester near the valve is one of the common methods.

Its simple method


Although there are different types of water hammer arrestor, they all have the same working principle. Their working principle is to employ a measured, compressible cushion of air or gas which is permanently separated from the water.

When a valve closes abruptly, the moving water column is displaced up into the arrester, compressing the permanent air charge until all the momentum of the moving water is safely dissipated, allowing the column to slow down calmly, preventing the development of water hammer.


In term of energy conservation, it changes the kinetic energy into the work done during compression of the air cushion.

1.1.2 Type of water hammer arrestor

1.1.2 Important location of water hammer arrestor in high rise building


In the high rise building, the sub-contractor must installs the water hammer arrestors between the valve and the pump.

As the place is of the highest pressure in high rise building, it would be the greatest chance to produce water hammer.


Besides, the most important thing is that if the size of water hammer arrestor is incorrect, it cannot develop its function of preventing water hammer.


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


As the size of the water hammer arrestor is very important, there are calculation step.....

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Step 2: Calculate the kinetic energy during the pump start


As the working principle of the water hammer arrestor is to convert the kinetic energy produced by water hammer to the work done during the compression of air cushion inside the water hammer arrestor, we should calculate the


Total mass of water in pipe:

Kinetic energy produced:

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


Prior to the step 3, we should study more than about the working concept of the water hammer arrestor.


There are three stage of the water hammer arrestor.


The first stage is pre-charge state.

In this stage, the water hammer arrestor contains designed volume of the air cushion that we want to find in the calculation. As water hammer arrestor is not installed in the pipeline, there is no water entering into the water hammer arrestor. Therefore, the pressure (P1) in the water hammer arrestor is not affected by the water pressure in the pipeline system and it is not the working pressure which help in convert the kinetic energy produced by water hammer into the work done.


The second stage is charge state.

In this stage, the water hammer arrestor is already installed into the pipeline system. As the water pressure is always higher than the designed pre-charge pressure, water even under normal condition in pipeline will enter the water hammer arrestor to compress the air cushion. This phenomenon occurs until the air cushion pressure is equal to the water pressure in the pipeline.

We called the pressure as the minim.....

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In the initial compression after installation of the water hammer arrestor, the V1 is changed to V2. As we set P1 is 100cm3, we can use the equation to find the value of V2:


During the main compression due to pump startup, we can use the equation to find the value of P3: P2V2n = P3V3n where n = 1.4 for air.


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):

During the energy conservation:

Kinetic energy= -work done


What we have found:

Work done per 100cm3 air

Kinetic energy in the pipe system

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 th.....

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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 .....

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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.....

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