Volume1 - Aventura

Cover Story
Pressure Boosting Hydropneumatic Tanks
Water Distribution
Water pressure bears a very important role in various applications whether it is at
the domestic level or in industry. The process of water distribution is similar for all
applications.
The easiest method of water distribution in a building or a complex comprises
of a source of water connected to a storage tank. All the water outlets, as per
requirements, are connected to the storage tank. The distribution pattern may vary
between a multi-storeyed building complex and a bungalow.
a. Multi-storeyed buildings: In case of a multi-storeyed building, the water is
initially stored in an underground tank of very large capacity. This water is
then pumped into multiple storage units placed on top of the buildings. It is
then distributed to individual floors under gravity.
b. Bungalows: In case of a bungalow, the water may be stored in an underground tank of very large capacity. This water is then
pumped into single or multiple storage units placed on top of the bungalow. It is then distributed to individual areas under gravity.
Alternatively, the water is pumped into the above-ground storage tank, directly from the source.
Water distribution under gravity: This is the most commonly employed technique for water distribution. As per the laws of physics, the pressure
generated in the water is directly proportional to the height from which it flows. Thus, water flowing from a very high location would have
higher pressure as compared to water flowing from a lower height, irrespective of the pipe size and dimensions. Thus, while designing a water
distribution system for a building, it is the height from which the water flows which will always determine the pressure of water at the outlet
points.
Shortfalls of the conventional systems: The conventional systems though effective have been proven to have several shortfalls, in current
times. These are listed below.
1. In a multi-storeyed building, although the height is great, locating the storage tanks on the topmost floor requires extensive design
consid erations to take care of the excess loadings. Moreover, in case of earthquakes, the buildings are more likely to collapse due to the
presence of these tanks as moving loads.
2. Also, in case of multi-storeyed buildings, the pressure generated in the pipeline at the lower levels is as high as that on the higher
floors. Thus, you have regular complaints of low pressure at various levels.
3. In case of bungalows, as the height is not much, the pressure is very low at the outlets, and in case multiple outlets are opened
simultaneously, the water pressure/flow further drops.
4. In this system, multi-level pumping is required in most cases, as the water pressure in the municipal mains is not sufficient to drive
the water up to the storage tanks provided on rooftops.
5. The pressure requirement for the new gadgets- like shower panels, Jacuzzis and dishwashers- employed these days is very high,
which conventional systems cannot meet.
Hydropneumatic tanks offer significant benefits over conventional systems, while solving common water
pressure problems.
Pressure Boosting systems have been an industry standard for over a couple of decades. They involve the setting
up of an external system to enhance the pressure in the pipelines/application circuit, as per the requirements of
the application. Pre-pressurised tanks have been an integral part of these systems for over twenty years.
Diaphragm tanks are preferred over conventional non-pressurised tanks because diaphragm tanks
separate air from water inside the tank, are pre-pressurised and require little maintenance.
Diaphragm tanks provide three times the draw down of water that conventional
tanks provide.
Pre-pressurised tanks are used in a variety of applications. They are being
extensively used worldwide for numerous applications such as:
• Pressure boosting in homes and buildings
• Industrial pressure boosters
• Reverse osmosis storage
• Irrigation
• Sprinkler systems
• Water hammer arresting
• Fire-fighting
• Heat expansion
• Thermal expansion
Pre-pressurised tanks or hydropneumatic tanks are used to provide pressure to very small public
water systems such as resorts, mobile home parks and very small communities. These tanks are
available in a variety of models and sizes.
Working of a Pressure Tank
The pressure-rated tank contains approximately two-thirds water and one-thirds air at full
capacity.
An air compressor is required to maintain proper volume of air within the tank at the necessary
pressure. At a low operating level, the tank will contain about one-third water and two-thirds
air. The air is pressurised to provide a system head and operates at about a 20-pound-per-square-inch pressure difference between high and
low water levels. A system using a hydropneumatic tank requiring an average operating pressure of 40-psi would then have 50 psi pressure at
high levels and a 30 psi pressure at low levels. In fact, these tanks with pre-pressurised water help to reduce the pump run time drastically in
applications that need small supplies of water at regular pressure over short intervals. The water stored in the tanks can be supplied at the preset
pressure without switching on the pump too frequently.
Hydropneumatic tanks are generally constructed of steel or fibreglass and must meet the standards of the American Society of Mechanical
Engineers (ASME) for pressure-rated tanks. The tanks are usually long and cylindrical, and positioned horizontally on concrete support piers.
They look similar to propane storage tanks.
A hydropneumatic tank consists of a diaphragm made up of a food-grade EPDM rubber membrane that separates the air from the water inside
the tanks. The diaphragm can also be a complete balloon where the air is inside the balloon bag and the water is trapped around the balloon and
the tank shell.
Selecting the Right Hydropneumatic Tank
The sizing of the tanks is based on Boyle’s Law. The effective draw down from the tank can be determined using the cut-in & cut-out pressure
required by the given system.For sizing a suitable boosting system, we need the following inputs :
• Capacity- flow rate in lph or m3/hr.
• Total volume- flow rate in lpm x pump runtime
• Select pump runtime
• Pump cut-in pressure- P1
• Pump cut-out pressure- P2
• Calculate tank efficiency using Boyle’s Law.
• Divide actual flow rate by tank efficiency.
Showing this calculation through an example, if
Pump flow rate = 10 litres per minute
Pump runtime = 1 minute
Total volume = Pump flow rate x pump runtime = 10 lpm x 1 minute = 10 litres
Cut-in pressure = P1 = 30 psi + 14.7 psi= 44.7 psi
Cut-out pressure = P2 = 50 psi + 14.7 psi= 64.7 psi
According to Boyle’s Law, tank efficiency = (P2 – P1)/P2
= (64.7– 44.7)/64.7= 0.309
Tank size = Total volume = 10.00 = 32.36 litres
Tank efficiency 0.309
Benefits of pressure systems
• An HPN tank extends the life of the pump
and enhances performance of the pump by
reducing the number of cycles.
• HPN tanks provide a closed drinking water
system. A closed system inhibits bacterial
and vermin intrusion. Conventional tanks
used in a gravity system are vented and
allow impurities to enter the system.
• HPN tanks help deliver equal pressur
throughout multi-level dwellings. Coventional
tanks create pressure disparities in homes.
• HPN tanks draw water from wells or cisterns
as needed and allow source replenishment.
Thus, from the above calculations, we can select a tank having a total volume of 32.36 litres,
so that at any given point of time it can deliver at least 10 litres of water, without switching on
the pump. The only other precaution to be taken is that an adjustment factor for the atmospheric
pressure must be considered while performing the calculations. The atmospheric pressure is
considered to be 1 bar or 14.7 psi. Similarly, the pump should also be selected keeping in mind the
atmospheric pressure.
Components Are Everything
Components Are Everything
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