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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


.....[read full text]

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The bulk modulus (k) of water is 2.19x10^9 Pa however this assumes that the water has no air bubbles in it. Often microscopic size bubbles can be seen suspended in the fluid. This can make a significant difference to the effective bulk modulus and so to the sonic speed. Often with water hammer sub atmospheric pressure and cavitation can also occur (as explained below).

This can liberate dissolved air from the water which forms air bubbles reducing the effective bulk modulus and so reducing the wave speed.


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 minimum working pressure (P2).


The third stage is maximum compression state. In the stage, the water arrestor is already installed into the pipeline system. The water pressure is the highest in the pipeline due to pump startup. Therefore, the water will compress the air cushion abruptly in largest volume until the balance between the air cushion and the water in pipeline is reached. We called the pressure in this stage as the maximum working pressure (P3).


Besides, the compression from P2 to P3 is used to convert the kinetic energy produced by water hammer into the work done for eliminating the water hammer.


Therefore, according to the working concept of the water hammer arrestor, we can find the three state pressure of the water arrestor.


P2 =minimum working pressure= the absolute pressure of the pipeline system


P1 = pre-charge pressure=P2 x λ whereλdepend on the design of the product.


P3 =maximum working pressure=the absolute pressure of the pipeline system + water hammer pressure rise (calculated in the step 1)


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


As we found all of the pressure in the three stage of water hammer arrestor, we can use the Boyle’s law to find the volume in these three stages:

As we cannot find the three stage volume by the practical method and the three value is unknown, we need to assume the pre-charge volume of air cushion (V1) is 100cm3 first.


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 maximu.....

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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 redu.....

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


With reverse flow eliminated, the forces necessary to produce water hammer on both the upstream and downstream sides of the valves are substantially eliminated.


1.3.2 Type of silent check valve


A check valve is a backflow prevention device: it allows flow in only one direction. The pressure of forward fluid flow opens the valve disc and the pressure from backflow closes it, forcing the disc against the valve seat. Three types of check valves are available to achieve this checking effect.


♦Swing Check Valve

A hinged disc assembly is suspended from the body to allow it to move freely. This configuration minimizes pressure loss and eases fluid flow. Swing check valves can be used for either horizontal or vertical (fluid flowing upward) pipe layouts.

♦Lift Check Valve

The structure of the lift check valve is the globe valve without the handwheel and any parts which related to manual operation. And have the cover in lieu of the bonnet. Because of its large fluid resistance, this valve is used primarily for small-bore applications.

♦Wafer Check Valve

Thanks to its wafer shaped design, this swing check valve is far thinner and lighter than conventional water hammer absorbing check valves. It features a built-in bypass circuit and superior closing action and is multifunctional high performance water hammer absorbing check valve.


1.3.2 What .....

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This can reduce the sudden change of velocity, preventing the water hammer from occurring in the check valve.



1.4 Cause 4: Air trapped in the pipeline


1.4.1 Source of air

In the high rise buildings, the pipeline containS much air before pump start-up. A large percentage of air will become trapped at system high points. To totally fill a pipeline with water, it is necessary to bleed off this air. As the pipeline fill with the water, some of the air will be pushed downstream to be released through hydrants, faucets, etc. However, a large amount will become trapped at system high points.

This phenomenon will occur because air is lighter than water and hence it will accumulate to the high points (Figure One). These phenomena will occur because air is lighter than water and therefore it collects in the high points to the high point.

Besides, a pipeline is never without air and typically the volume is substantial. It is because air is continuously forced into the system by pump and air is drawn in through packing, valves under vacuum conditions.

1.4.2 Impact of air on high rise building


When the pump starts up, the flow velocity will increase which in turn make the air pockets in the high points dislodge and be then stopped by another high point. This process would result the sudden change of flow velocity which can lead to a high pressure water hammer. Serious damages to valves, fittings, gaskets, or even breakage of the line can occur.

As the source of the air is permanent, we should use long term solutions to eliminate .....

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  1. 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.....

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