开启阅读模式

WATERHAMMER

A COMPLEX PHENOMENON WITH A SIMPLE SOLUTION

Waterhammer is an impact load that is the most misunderstood force known to pressure transducers today.
A waterhammer is created by stopping and/or starting a liquid flow suddenly. The results of a waterhammer or impulse
load are devastating to a pressure sensor. The impulse load occurs suddenly, in the millisecond time frame, but the effects
of it last a life time. Waterhammers occur in almost all pressure systems and usually can not be stopped without extensive
time, energy and studies.

A common example of a waterhammer occurs in most homes everyday. Simply turning off a shower quickly sends
a loud thud through the house; this is a perfect example of a waterhammer. Dishwashers and washing machines make these same
sounds, because inside them small solenoid valves are being opened and closed quickly, producing this pulse noise. The key
phrase in the examples above was turning on or off the water "quickly" verses turning it off slowly. In the shower example,
if you turn the water off slowly, the waterhammer will not occur. Common industrial hardware like relief valves, solenoid
valves, valves in general, centrifugal pumps, positive displacement pumps, and regulators can and will cause heavy hammer effects.
A simple solution to this devastating effect is to protect each sensor with a pressure snubber. Snubbers are low ticket items
that will insure that this hammer effect will not render your costly sensor useless. All pressure sensors should utilize snubbers
for all installations.

The hammer occurs because an entire train of water is being stopped so fast that the end of the train hits up
against the front end and sends shock waves through the pipe. This is similar to a real train, instead of slowing to a stop,
it hits into a mountain side. The back of the train continues forward even though the front can not go anywhere. Since the water
flow is restricted inside the pipe, a shock wave of incompressible water travels back down the pipe deflecting everything in its path.
An unprotected transducer in the path of this monstrous wake is without question, going to sustain heavy damage.

To understand the damage caused by the waterhammer forces, it is necessary to understand the principles behind
the sensor. Most pressure sensors utilize a rigid diaphragm as the primary sensing element. The diaphragm deflects due to the pressure,
and its deflection is transformed to an electrical output via various methods. The key component is the rigid diaphragm. The rigid
diaphragm deflects only on the order of a thousandth of an inch. With a large wake of fluid hitting the sensor, it is no wonder the
diaphragm is bent beyond its elastic limit and permanent damage is done. Remember that a snubber eliminates this effect and therefore
should always be installed on every pressure system.

Snubbers are chosen by the media that they will be used on such as liquids, gases or dense liquids like motor oils,
and their physical mounting fittings. Snubbers only let so much fluid pass through per unit time, eliminating the surge from hitting
the diaphragm. Liquids possess a large hammer effect because they are incompressible, but gases can also possess a hammer effect
large enough to render a sensor useless. A practical analogue to a snubber is a sponge in the drain of a sink. The sponge ensures
that the sink empties slowly, instead of all at once. A lot of common questions are asked about hammer effects; the following
are just a few.

WILL A SNUBBER AFFECT THE RESPONSE TIME OF MY PRESSURE trANSDUCER?

In most cases, the transducer is connected to a meter of a recorder that updates at 2 to 3 times a second;
therefore a snubber will not affect it at all.

WHAT ARE THE SYMPTOMS THAT MY SENSOR HAS BEEN DAMAGED BY A FLUID HAMMER?

Most sensors will exhibit a higher than normal output at zero pressure (a zero shift). This occurs because
the diaphragm can not return to zero. In severe cases no output occurs or the output does not change with an
increase in pressure.

IF MY SENSOR HAS A LARGE ZERO OFFSET CAUSED BY THIS HAMMER EFFECT CAN IT BE REPAIRED?

Most sensors are non-repairable. The diaphragm is the main building block of the sensor. When building
a sensor the diaphragm is first built and the all the other components are chosen to achieve the rated specification.
When a diaphragm bends beyond its elastic limit, it can not be bent back to original shape or replaced because of the unique
components associated with the original diaphragm. If a diaphragm does have a slight zero shift, less than 10%, it probably
is still linear and can be used. Before reinstalling it in the system, please acquire a snubber or the hammer effect will
occur again and possibly damage the unit further.

WILL A SNUBBER STOP AN OVERPRESSURE?

Snubbers stop spikes only, they do not perform miracles. An overpressure will not be stopped a snubber.
A spike lasts only on the order of milliseconds; any overpressure for more than that time will damage the sensor.

HOW IS A SNUBBER INSTALLED IN A PRESSURE SYSTEM?

The snubber would screw on to the front end of the transducer and then thread into the piping system.
The snubber is located between the piping under pressure and the pressure transducer. The following brief equations
summarize the hammer effect and is followed by an example of the destructive force of waterhammer. The following equation
determines the maximum pressure change that occurs during a fluid hammer. The equation assumes that the piping is inelastic.

ΔP=rcΔv/g
where
c for liquids =(Eg/r)½
and
c for gases =(KgRT)½

WHERE

P is the change in pressure resulting from the fluid hammer (pounds per square foot)
r is the fluid density (pound mass per cubic foot)
cis the speed of sound in the fluid (feet per second) v is the change in velocity of the fluid (feet per second)
g is the gravitational constant (32.2 feet per second per second)
E is the bulk modulus of the fluid media (listed in PSI but must be converted to PSF)
k is the ratio of specific heats (k = 1.4 for air)
R is the specific gas constant (foot pounds per pound mass per degree Rankine)
T is the absolute temperature in Rankine

Example of waterhammer occurring in typical house piping. Assuming you have one inch water piping, how
much of a change in pressure will be created from a waterhammer?

Assume that the water is flowing in 10 gallons per minute and the temperature is about room temperature
(70°F). A 1 inch schedule 40 pipe has an internal area equal to 0.00600 ft2.

Fluid velocity V = Q/A = 10 gpm (1/448.83 gpm/cfs)/.006ft2 = 3.71 ft/sec.

Where Q is the flow rate, and A is the internal area in the pipe.

In this example, a 1 inch pipe with a flow rate of 10 gpm had a hammer effect resulting in an increase
in pressure of 243 PSI above normal operating conditions. Considering normal city water pressure of 50 PSI, most end
users would select a sensor of approximately 100 PSI full scale to be on the safe side. A 100 PSI sensor usually has
an over pressure of 200% associated with it, meaning it will be able to withstand 200 PSI. Now the hammer increases the
system from 50 PSI to 293 PSI (50 + 243), which is overpressurizing the transducer and causing damage to it. Most end users
are puzzled as to how a system that is supplied with only 60 PSI is capable of producing over 200 PSI. After reading this
article it should be evident that fluid hammers are a complex phenomena with a simple solution: installing snubbers on all
pressure transducers.

c=(Eg/r)½=[(320x10³lbs/in²)(144in²/ft²)(32.2ft/sec²)/62.3lb/ft³]
½=4880ft/sec

ΔP=rcΔv/g=(62.3lb/ft³)(4880ft/sec)(3.71ft/sec)/32.2ft/sec²=35,029lbs/ft² or 243lbs/in²

PROPERTIES OF WATER AT ATMOSPHERIC PRESSURE

Temp. Density Density Kinematic
Viscosity
Viscosity Surface
Tension
Vapor
Pressure
Bulk
Modulus
°F lbm/ft3 slug/ft3 lbf-sec/ft2 ft2/sec lbf/ft Head
ft
lbf/in2
32 62.42 1.940 3.746
EE-5
1.931
EE-5
0.518
EE-2
0.20 293
EE3
40 62.43 1.940 3.229
EE-5
1.664
EE-5
0.514
EE-2
0.28 294
EE3
50 62.41 1.940 2.735
EE-5
1.410
EE-5
0.509
EE-2
0.41 305
EE3
60 62.37 1.938 2.359
EE-5
1.217
EE-5
0.504
EE-2
0.59 311
EE3
70 62.30 1.936 2.050
EE-5
1.059
EE-5
0.500
EE-2
0.84 320
EE3
80 62.22 1.934 1.799
EE-5
0.930
EE-5
0.492
EE-2
1.17 322
EE3
90 62.11 1.931 1.595
EE-5
0.826
EE-5
0.486
EE-2
1.61 323
EE3
100 62.00 1.927 1.424
EE-5
0.739
EE-5
0.480
EE-2
2.19 327
EE3
110 61.86 1.923 1.284
EE-5
0.667
EE-5
0.473
EE-2
2.95 331
EE3
120 61.71 1.918 1.168
EE-5
0.609
EE-5
0.465
EE-2
3.91 333
EE3
130 61.55 1.913 1.069
EE-5
0.558
EE-5
0.460
EE-2
5.13 334
EE3
140 61.38 1.908 0.981
EE-5
0.514
EE-5
0.454
EE-2
6.67 330
EE3
150 61.20 1.902 0.905
EE-5
0.476
EE-5
0.447
EE-2
8.58 328

EE3

160 61.00 1.896 0.838
EE-5
0.442
EE-5
0.441
EE-2
10.95 326
EE3
170 60.80 1.890 0.780
EE-5
0.413
EE-5
0.433
EE-2
13.83 322
EE3
180 60.58 1.883 0.726
EE-5
0.385
EE-5
0.426
EE-2
17.33 313
EE3
190 60.36 1.876 0.678
EE-5
0.362
EE-5
0.419
EE-2
21.55 313
EE3
200 60.12 1.868 0.637
EE-5
0.341
EE-5
0.412
EE-2
26.59 308
EE3
212 59.83 1.860 0.593
EE-5
0.319
EE-5
0.404
EE-2
33.90 300
EE3


WATERHAMMER: A COMPLEX PHENOMENON WITH A SIMPLE SOLUTION的更多相关文章

  1. Building simple plug-ins system for ASP.NET Core(转)

    Recently I built plug-ins support to my TemperatureStation IoT solution web site. The code for .NET ...

  2. [Javascript + rxjs] Simple drag and drop with Observables

    Armed with the map and concatAll functions, we can create fairly complex interactions in a simple wa ...

  3. [转] Creating a Simple RESTful Web App with Node.js, Express, and MongoDB

    You can find/fork the sample project on GitHub Hey! This and all my other tutorials will soon be mov ...

  4. Tomcat – java.lang.OutOfMemoryError: PermGen space Cause and Solution

    Read more: http://javarevisited.blogspot.com/2012/01/tomcat-javalangoutofmemoryerror-permgen.html#ix ...

  5. PID控制器(比例-积分-微分控制器)- I

    形象解释PID算法 小明接到这样一个任务: 有一个水缸点漏水(而且漏水的速度还不一定固定不变),要求水面高度维持在某个位置,一旦发现水面高度低于要求位置,就要往水缸里加水. 小明接到任务后就一直守在水 ...

  6. External Configuration Store Pattern 外部配置存储模式

    Move configuration information out of the application deployment package to a centralized location. ...

  7. {ICIP2014}{收录论文列表}

    This article come from HEREARS-L1: Learning Tuesday 10:30–12:30; Oral Session; Room: Leonard de Vinc ...

  8. Optimizing Performance: Data Binding(zz)

    Optimizing Performance: Data Binding .NET Framework 4.5 Other Versions   Windows Presentation Founda ...

  9. Code Simplicity–The Science of Software Development 书摘

    Chapter1 Introduction That is the art and talent involved in programming—reducing complexity to simp ...

随机推荐

  1. Android事件总线

    Android中Activity.Service.Fragment之间的相互通信比较麻烦,主要有以下一些方法: (1)使用广播,发送者发出广播,接收者接收广播后进行处理: (2)使用Handler和M ...

  2. [python] CSV read and write using module xlrd and xlwt

    1. get data from csv, skip header of the file. with open('test_data.csv','rb,) as csvfile: readCSV = ...

  3. java单例模式的实现方式

    一.什么是单例模式 单例:保证一个类仅有一个实例,并提供一个访问它的全局访问点. 单例模式是一种常用的软件设计模式之一,其目的是保证整个应用中只存在类的唯一个实例. 比如我们在系统启动时,需要加载一些 ...

  4. C/C++内存泄漏及检测

    参考 http://www.cnblogs.com/skynet/archive/2011/02/20/1959162.html

  5. 建造者模式组装mybatis参数Example()

    参考:github,   https://github.com/liuxiaochen0625/MyBatis-Spring-Boot-master.git 从controller组装tk.mybat ...

  6. 也许你需要点实用的-Web前端笔试题

    之前发的一篇博客里没有附上答案,现在有空整理了下发出来,希望能帮助到正在找工作的你,还是那句话:技术只有自己真正理解了才是自己的东西,共勉. Web前端笔试题 Html+css 1.对WEB标准以及w ...

  7. JDBC数据库访问操作的动态监测 之 Log4JDBC

    log4jdbc是一个JDBC驱动器,能够记录SQL日志和SQL执行时间等信息.log4jdbc使用SLF4J(Simple Logging Facade)作为日志系统. 特性: 1.支持JDBC3和 ...

  8. android Animation介绍

    Animation介绍: 在Android SDK介绍了2种Animation模式: 1. Tween Animation:间动画,通过对场景里的对象不断做图像变换(平移.缩放.旋转)产生动画效果,即 ...

  9. Swift3 - String 字符串、Array 数组、Dictionary 字典的使用

    Swift相关知识,本随笔为 字符串.数组.字典的简单使用,有理解.使用错误的地方望能指正. ///************************************************** ...

  10. 看看C# 6.0中那些语法糖都干了些什么(终结篇)

    终于写到终结篇了,整个人像在梦游一样,说完这一篇我得继续写我的js系列啦. 一:带索引的对象初始化器 还是按照江湖老规矩,先扒开看看到底是个什么玩意. 1 static void Main(strin ...