Head

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May 21st 2007

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NEWSLETTER FOR MAINTENANCE COMMUNITY Word for the day: HEAD

Head is the most commonly used term in selecting and specifying pumps apart from many other uses. For example, to pump water for a 10-storey apartment, we will select a pump with certain “head” apart from other specifications. Head is defined as the amount of pressure exerted by a column of liquid at any given point, and is directly proportional to its height. To understand its meaning and go further, let us consider a simple example, as shown in figure 1. Drill 3 holes at different heights in a pipe of approximately 3 feet length at equal distances and plug them. Fill the pipe completely with water and now open all the three plugs together. You will observe that water from bottom hole will flow faster than from top hole. This happens due to the pressure exerted by weight of water, which differs from point to point and is directly proportional to the level. This level is called Head and is dependent on density of the fluid being used. Density is defined as weight per unit volume (kg/m3). For example, density of ordinary water is 1000 kg/m3 where as for Petrol it is 790 kg/m3 and hence for a given level water exerts more pressure than petrol. Since pressure and head are Figure 1 related to each other, let us understand the conversion using the following formula. p (pressure, kg/m2) = σ (density, kg/m3) x h (liquid level or head, meters) To exert 1 kg/cm2 of pressure, let us now calculate the level of water and petrol required. p = 1 kg/cm2= 1 x 100 x 100 kg/m2

p = 1 kg/cm2= 1 x 100 x 100 kg/m2

σ, density of water = 1000 kg/m3

σ, density of petrol = 790 kg/m3

10000 kg / m2 = 1000 kg / m3 x h

10000 kg / m2 = 790 kg / m3 x h

h = (10000/1000) = 10 meters

h = (10000/790) = 12.66 meters

To exert a pressure of 1 kg /cm2, water should be filled to 10 meters where as petrol should be filled to 12.66 meters. Apart from water, mercury (Hg) is also used as standard for low pressure. To exert same 1 kg / cm2, only 76 centimeters (760mmHg) of mercury is sufficient. Human blood pressure is measured in mmHg and for a healthy person it is 120/80. During pumping of blood, heart develops a head of 120mmHg, which is equivalent to 0.16 kg / cm2 and during resting, it develops a head of 80mmHg or 0.1 kg / cm2. Head value is the most important parameter in selecting a pump for certain application. For a pump, head is defined as the vertical height to which it can throw the fluid when left freely. Normally, the pressure delivered by pump will change when density changes and head almost remains same. Since various units are used in specifying pressure, following table provides a general overview as reference. For example, 1 Bar is 1.097 kg / cm2 which is 14.504 Pound per square inch.

Pascal (Pa) 1 Pa 1 bar 1 atm psi

Bar (bar)

≡ 1 N/m² 10−5 100 000 ≡ 106 dyn/cm² 101 325 1.01325 6 894.76 68.948×10−3

Technical atmosphere Atmosphere (at) (atm) 10.197×10−6 1.0197 1.0332 70.307×10−3

Torr (mmHg)

9.8692×10−6 7.5006×10−3 0.98692 750.06 ≡ 1 atm 760 −3 68.046×10 51.715

Pound-force per square inch (psi) 145.04×10−6 14.504 14.696 ≡ 1 lbf/in²

If you like to improvise this article or contribute or comment please mail us at: [email protected] This document contains information for reference only. We assume no responsibility for its implication.