Pressurised vs Towers Advantages/Disadvantages 4

[stanier]

What are the advantages and disadvantages when comparing a pressurised system versus a elevated tower system for water distribution?

I have considered that

1)sizing pumps for fire duty is one disadvantage for the former.
2)use off peak energy versus off peak energy is a disadvantage of the former.
3) need to size pumps for flushing?

Do members have any other ideas?

 

[bimr]

1. Emergency power requirements are greater for a pumped system. Fire pumps will take a larger power unit.

2. Capital costs will be higher for a water tower system.

3. It is easier to control system pressure with an elevated tower.

4.

 

[mflam]

Seems to me that the only advantage of an all-pumped system is the first cost. You have to store the water somewhere for your peaks. If you can build an elevated tower you should.

 

[RWF7437]

It has been well over two centuries now since gravity was last turned off for non-payment or any other reason.

 

[stanier]

My problem is that a very well known USA consultant is peddling the idea in Southern Asia that pressurized systems save up to 30% in energy costs. This they claim is achieved by pumping at reduced pressure in times of low demand. Of course some users will be disadvantaged.

Personally I think the consultant is adopting this approach as a pressurized system requires more engineering at the expense of reliability.

There is a very good report from the Asian Development Bank that criticizes conventional wisdom in bringing water to the urban poor in Southern Asia. The consultant seems more imterested in there own livelihood than the poor.

 

[RWF7437]

Gravity works the same in Asia as it does elsewhere on this planet. It also costs the same; namely ZERO. Pumping costs energy and money; in Asia, North and South America, Europe and islands in the Oceans. Only in Eutopia is it free.

Sounds to me like your well known consultant should be investigated by the client countries to whom this kettle of fish is being sold.

You may have to demonstrate the obvious to thos countries to unmask these guys.

good luck

 

[hydrae]

A poorly designed gravity system may require more energy but only if pumps and piping are not optimized for efficiency.

Gravity always works.

No control system (no mater how well tuned) can keep pressure as smooth and flow as responsive as gravity in a large at service level tank.

If reduced pressure is allowed at low demands nothing says you have to have just one tank, using two tanks at different elevations in the same control scheme (as the pumps would have been engaged at) will save even more power.

Hydrae

 

[semo]

I would not necessarily agree with a blanket statement "A pressurized system will save 30% in energy costs". It all depends on the system demand and the pump(s) sizing.

Given the pumps for both types of systems have the same efficiency, the energy (KwH) needed "(Q*H/%eff)*.746*Time" for one days useage doesn't matter if the pump rate is 1000 gpm or 500 gpm. It will be the same.

A proper gravity system will have a greater storage available for extended use; therefore, a smaller pump sized for longer peaks can be used to fill the storage over a longer time.

The pressure system has less storage and requires a larger pump that can meet the shorter peak useages which will be higher (ie. An hourly peak use will be higher than a daily peak use).

The increased start/stop of the pumps will reduce their and their controls (starters) life expectancy thereby increasing the maintenance and replacement costs.

If gravity systems are bad and inefficient, why do all large and almost all smaller municipalities use them?

 

[JStephen]

In Texas, the minimum pressurized/ elevated storage capacity per connection is mandated by state rules. IE, you don't have a no-storage option, you either have hydropneumatic tanks for pressurized storage (typical on small systems) or elevated tanks. I'd be a little hesitant to use the argument that "it must be good because everyone does it this way", when here, at least, everyone is required to do it this way.

I'm not a water system designer, but I would assume that the extent to which people water yards (everyone, all summer around here) and the extent to which water is expected to be used for fire protection would influence the decision.

 

[BRIS]

With a simple pressurised system the pumps maintain a fixed pressure head at the point where they pump into the system. If The pressure drops (high demand) then the next pump starts and so on (fixed speed pumps and pressure switches). The pump capacity must be greater than the peak demand capacity. .

With a tower you have a set of pumps that pump into the tower - fixed head - if one pump cannot maintain the tank full then the next pump starts etc. At peak demand storage in the tower contributes to demand and total pump capacity may be less than peak demand. Pump costs are less with a tower!

It would appear that your consultant is proposing a much more sophisticated operating system with the pumps adjusting to the system curve - lower pressure in the system at low demand - variable speed pumps??. In this case there may be significant savings in power and also reduced leakage losses but I would guess that the solution is too sophisticated and not appropriate but It is not possible to give a judgement without some knowledge of the overall system.