Bladder Surge Tanks Inroduction

Pressure transients (Surges)

Pressure transient is a phenomenon which can be described as waves propagated in both positive and negative magnitude. These transients are caused by rapid change in flow velocity accompanied by a change in pressure. Rapid change in flow velocity may
be caused by events such as pipe bursts, sudden changes in demand, sudden pump start-ups and shut-offs, opening and closing of fire hydrants, quick closing and opening of in-line isolating valves, flushing and draining operations, fire flow, feed tank draining, and other similar events.
Control of pressure transients should be one of the main concerns of the hydraulic engineer during the process of system design.
Pressure transients can cause cracks and unseen small breaks in buried pipelines, pipe fittings and accessories and also can cause severe pipe ruptures and pipe bursts.

Bladder Surge Vessel

A Bladder Surge Vessel is a surge control device installed with a butyl rubber bladder. A pre-charge pressure is calculated to give the required elasticity to push the water into the system following a pump trip. As there is no contact between the compressed air
and the water, there is no dissolution. Thus, there is no requirement for a permanent regulation system including compressors, etc.
Once the vessel has been commissioned and the correct pre-charge has been introduced, the vessel will operate automatically, emptying when called upon and refilling with the return waves until naturally reaching its steady state balance.s in elevation between the ends.

Bladder Vessel Concept

The vessels can be installed either horizontally or vertically.
The vessels are treated internally with food quality epoxy paint for corrosion protection and externally according to client specification.
In order to verify the water level in the vessel, it is normally equipped with a level indicator as well as a manometer to verify the initial pre-charge pressure.
If it is hydraulically required, the vessel is equipped with a non return valve or an incorporated bypass in order to reduce the over pressure by consuming the energy of the flow reversal.

Operation

1. Initially, the pre-charge pressure must be adjusted to the value resulting from the hydraulic analysis (pre-charge can be either compressed air or nitrogen). At this stage, the bladder contains no volume at all.
2. When the gate valve is opened, the water will enter the vessel under static conditions and begin to compress the gas (static pressure is always higher than pre-charge pressure).
3. The water entering the vessel will further compress the pre-charged gas until a balance between the liquid and the compressed gas is reached.
4. Immediately after a pump trip, the pressure in the line will start to decrease and the elastic energy in the vessel will cause the discharge of water from the vessel into the line. This prevents dangerously low pressure along the pipe.
5. As the pressure may become very low, the flow will reverse. Water will then enter into the vessel via a reduced diameter (drilled non return valve or bypass) if hydraulically required. Several oscillations may occur before static state is reached.
6. When the pump restarts, the vessel will continue to fill until dynamic steady state is reached and it is then once again prepared for the next pump trip.

Water Pressure Booster - Hydrofort

If a pumping station is called upon to pump smaller volumes than those resulting from continuous operation, a hydropneumatic accumulator is used to ensure an intermittent operation of the pumps. During the period of decompression of the accumulator, the pumps are stopped.

Subsequently, a pump will start up and operate long enough to fill the hydropneumatic vessel. The study to define the volume of this hydropneumatic accumulator is carried out using numerical simulations of operation during a cycle.

The essential information for optimum performance are:

• the pump curve characterising the pressure/flow of the “first” pump at nominal speed (jockey pump or the smallest of the principal pumps).
• the number of permutable identical pumps.
• the admissible frequency of pump starts (electrical motor).
• the adjustment of the pressostat for the start and stop of the pump.
• altimetric levels of the pumps, the accumulator and of the pressostat.

The results of a calculation testing the volume of a Hydrofort with a given precharge pressure are:

• the length of operation of the pump and the length of the decompression of the accumulator with the pump stopped.
• the frequency of start ups obtained.

In an installation, the duration of the pumping cycles is a function of the flow consumed by the network. A critical flow rate exists, causing a maximum frequency of start-ups. For this flow rate, the length of operation of the pump equals the length of the duration of the emptying of the useful volume stored in the accumulator.
On no account can the accumulator be considered as surge protection, on the contrary... it can aggravate the transient pressure oscillations because it causes a sudden cut in the flow rate (almost instantaneous and more violent than when the pumps are stopped) when it is completely empty and has attained its dry precharge pressure.

The surge function, studied using a more general specific hydraulic analysis, should be handled by using a second hydropneumatic vessel that has been sized to feed the network during the deceleration from steady state to static state, which is often characterised by low pressure. The precharge pressure of the surge vessel is therefore very low. Moreover, the necessary flow rates of the 2 vessels have different maximum values, which justify 2 different connection diameters. Normally, in a station involving these 2 functions, the accumulator will have a large volume, a high precharge pressure and a small diameter connection, whilst the surge vessel will have a small volume, a low precharge pressure and a large diameter connection.

Bladder Surge Tanks Hydrochoc - Vertical Anti water hammer with food quality reinforced bladder 1000 to 20,000 L
Bladder Surge Tanks Hydrochoc - Horizontal Anti water hammer with food quality reinforced bladder 500 to 50,000 L
Water Pressure Booster Hydrofort - Vertical 1000 to 20,000 L
Water Pressure Booster Hydrofort - Horizontal 500 to 50,000 L