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# Vincent T. H. CHU

Why are axial flow pumps suitable for large flows and low heads?

It is well known that axial flow pumps are most suitable for providing large flows and low heads. The reason behind this is closely related to the configuration and design of the pumps. In axial flow pumps, the size of inlet diameter is greater than that of impeller diameter. For low flow condition the velocity is relatively small and this increases the chances of occurrence of separation which brings about additional head losses and vibration. On the contrary, if the discharge is large enough the problem of separation is minimized.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Why are radial flow pumps suitable for small flows and high heads?

In radial flow pumps, a diffuser/volute is normally designed at it outlet to convert the kinetic energy gained during the pumping process to pressure head. The diffuser is characterized by widening of outlet pipes, resulting in the decrease of velocity (by continuity equation) and an increase in pressure head (by Bernoulli’s equation). In case of large flows to be handled by the pumps, the large velocity results in formation of significant Coriolis force which tends to deviate the outlet flow from design conditions.

At the inlet part of the pumps, the inlet size is smaller than the diameter of the impeller. Consequently, the velocity of flow associated with a small area is relatively large and there is less problem of separation in low flow condition. All in all, the efficiency of radial flow pumps is high when handling small flows.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What is the purpose of adding cooling pipes or even using cold water for concrete in concreting operation?

All these measures aim at reducing the placing temperature and reducing thermal cracks induced during concreting of massive pours. Since the final concreting temperature should be the ambient temperature, reducing the initial placing temperature will also lower the peak hydration temperature. Therefore, the temperature difference between the hydration peak and the ambient temperature is reduced accordingly and subsequently the thermal effect to concrete structure can be reduced by controlling the placing temperature.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

In selection of waterstop, shall engineers use plain dumb-bell type or center-bulb type?

The plain dumb-bell type is used for joint location where small movements are anticipated. Therefore, construction joints are desirable locations of this type of waterstop. On the other hand, center-bulb type waterstop is suitable for expansion joints or locations where lateral and shear movements occur due to settlement or deflection. Reference is made to W. L. Monks (1972).

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

In the design of service reservoirs, how are reservoir floors designed to prevent leakage of water due to seasonal and shrinkage movements?

There are in general two main approaches in designing floors of service reservoirs:

(i) In the first method, movement joints are designed in each panel of reservoir floors so that they can expand and contract freely. Each panel is completely isolated from one another and a sliding layer is placed beneath them to aid in sliding.

(ii) The second method, on the contrary, does not make provision to free movement. Due to seasonal and shrinkage movements, cracks are designed to occur in the reservoir floors such that very tiny cracks are spread over the floor and these cracks are too small to initiate corrosion or leakage. However, in this case, the amount of reinforcement used is much larger than the first approach.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Should mild steel or high yield steel be adopted as reinforcement of water-retaining structures?

In designing water-retaining structures, movement joints can be installed in parallel with steel reinforcement. To control the movement of concrete due to seasonal variation of temperature, hydration temperature drop and shrinkage etc. two principal methods in design are used: to design closely spaced steel reinforcement to shorten the spacing of cracks, thereby reducing the crack width of cracks; or to introduce movement joints to allow a portion of movement to occur in the joints.

For the choice of steel reinforcement in water-retaining structures, mild steel and high yield steel can both be adopted as reinforcement. With the limitation of crack width, the stresses in reinforcement in service condition are normally below that of normal reinforced concrete structures and hence the use of mild steel reinforcement in water-retaining structure will suffice. Moreover, the use of mild steel restricts the development of maximum steel stresses so as to reduce tensile strains and cracks in concrete.

However, the critical steel ratio of high yield steel is much smaller than that of mild steel because the critical steel ratio is inversely proportional to the yield strength of steel. Therefore, the use of high yield steel has the potential advantage of using smaller amount of steel reinforcement. On the other hand, though the cost of high yield steel is slightly higher than that of mild steel, the little cost difference is offset by the better bond performance and higher strength associated with high yield steel.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

When designing a water storage tank, should movement joints be installed?

In designing water storage tanks, movement joints can be installed in parallel with steel reinforcement. To control the movement of concrete due to seasonal variation of temperature, hydration temperature drop and shrinkage etc. two principal methods in design are used: to design closely spaced steel reinforcement to shorten the spacing of cracks, thereby reducing the crack width of cracks; or to introduce movement joints to allow a portion of movement to occur in the joints.

Let’s take an example to illustrate this. For 30m long tanks wall, for a seasonal variation of 35 degree plus the hydration temperature of 30oC, the amount of cracking is about 8.8mm. It can either be reduced to 0.3mm with close spacing or can be absorbed by movement joints. Anyway, the
thermal movement associated with the seasonal variation of 35oC is commonly accounted for by movement joints.

For water-retaining structure like pumping stations, the crack width requirement is even more stringent in which 0.2mm for severe and very severe exposure is specified in BS8007. It turns out to a difficult problem to designers who may choose to design a heavy reinforced structure. Obviously, a better choice other than provision of bulky reinforcement is to allow contraction movement by using the method of movement joints together with sufficient amount of reinforcement. For instance, service reservoirs in Water Supplies Department comprise grids of movement joints like expansion joints and contraction joints.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

In the design of service reservoirs, horizontal reinforcement in walls of reservoirs is placed at the outer layer. Why?

Since service reservoirs are designed as water-retaining structures with stringent requirement of crack width control, the design of reinforcement of service reservoirs is under the control of serviceability limit state. For the walls of service reservoirs, contraction and expansion of concrete are more significant in the horizontal direction of walls because of their relatively long lengths when compared with heights. In this connection, in order to minimize the usage of reinforcement, horizontal bars are placed at the outmost layer so that the distance of reinforcement bars to concrete surface is reduced. Since the shorter is the distance to the point of concern, the smaller is the crack width and hence with such reinforcement arrangement advantages are taken if the reinforcement bars in the critical direction are placed closest to concrete surface.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What is the difference in arranging pumps in series and in parallel?

For identical pumps with similar functions, if the pumps arranged in series, the total head is increased without a change to maximum discharge. On the other hand, for pumps arranged in parallel to one another, the discharge is increased without any changes to maximum head.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What is the purpose of uniform rate of application of water in watertightness test of water retaining structures?

In water tightness test of water retaining structures, it normally requires the filling of water at a uniform rate or letting the water pool stand alone for some time before actual measurement is carried out. The reason for such provision in watertightness test is to allow sufficient time for water absorption to take place in concrete. After allowing sufficient time for water absorption to occur, the subsequent measurement of a fall in water level is deemed to be caused by water leakage instead of water absorption provided that adjustment has been made to other external effects such as evaporation.