A baffled hood design which dissipates a considerable amount of jet energy has been developed for water resources projects where space is at a premium
Ease of construction, minimum maintenance requirements and overall cost-effectiveness, are increasing the popularity of fixed cone valves for downstream control, flow regulation and energy dissipation under medium and high heads.
Typically, a fixed cone valve discharges conical spray into the atmosphere and dissipates part of the energy by interaction with, and entrainment of, air. However, large conical spray is not always acceptable due to various constraints, such as the potential for erosion in adjoining areas or the proximity of a power house and other structures.
In the past a variety of hoods have been used to confine water spread and spray and direct it into a suitable stilling pool or some other energy dissipating arrangement. Such arrangements contain spray but can not dissipate the high kinetic energy of the concentrated jet efficiently, resulting in significant problems due to scour and erosion downstream.
Furthermore, the energy dissipater needs to be excessively long in order to accommodate the trajectory of the high velocity jet issuing from the hood. This is especially true for valves operating under high heads. However, space restriction and economy often prevent the provision of such long dissipating basins.
At the Southern Peru Copper Corporation’s Torata river flood control project in Peru three fixed cone valves had to be housed in an underground chamber, and conventional hooded valves would have required a large length of the underground chamber to accommodate the exiting jet. When finalising the layout of the project the main objectives and challenges were to:
• Reduce the overall length of the underground chamber.
• Minimise vibrations during energy dissipation.
• Ensure tranquil flow into the exit tunnel downstream.
To achieve these objectives, ECI, the water resources division of US-based company DMJM+HARRIS, conceptualised a three-stage energy dissipation arrangement. The main components of this were the provision of the fixed cone valves with a baffled cylindrical hood around each valve to dissipate energy in a confined space, together with provision of a vertical impact wall downstream on which the issuing jet will impact.
This design required refinement and verification of its performance by means of hydraulic model studies. As a result, a study was completed by personnel at the Utah Water Research Laboratory (UWRL) to develop and assess the energy dissipating characteristics of the baffled hood to be used in conjunction with fixed cone valves.
The laboratory researched and improved on the hood design conceptualised by ECI to maximise jet energy dissipation and minimise back-splash and jet trajectory length. The results confirmed that 95% of the energy produced by the water discharging through the valve could be dissipated as it passed through the valve and baffled hood combination. In this way, the impact wall is subjected to very little impacting energy from the resulting jet, thus eliminating any concerns about vibration in the cavern, and ensuring tranquil flow downstream into the tunnel under all conditions of flows and heads. The laboratory also developed details to minimise back-splash, which is often a problem with such hoods. This design can provide benefits to owners of existing and new facilities where hooded fixed cone valve installations are employed.
Conventional fixed cone valves discharge a conical jet into the atmosphere with the energy being dispersed in the form of an aerated spray. For installations requiring the spray to be contained, hoods are generally designed to ensure there is a conical upstream section connected to a cylindrical spool. The conventional hood captures and turns the flow issuing from a fixed cone valve, which results in very little jet energy dissipation. The jet has a large amount of kinetic energy and travels a large distance downstream. To minimise the space requirements for dissipation of the jet’s energy downstream from the valve, a hood design that dissipates a considerable amount of jet energy was developed for the Torata river flood control project.
Basic hydraulic principles suggest that energy in an outlet stream can be dissipated by radically changing the direction of flow. This principle was used in the development of the baffled hood. The baffles force the flow to radically change directions in the cylindrical segment of the hood, resulting in a significant reduction of kinetic energy. The hood – which consists of a series of rows of baffles, staggered around the inner circumference of the hood – was provided to turn the jet onto itself in stages. This movement creates a large amount of turbulent dissipation that consumes a large quantity of energy, but results in an undesirable back-splash or return flow, a problem common even with conventional hood designs.
Provision of adequate space for air is also essential because of the large air demand required for effective operation of the fixed cone valves. In addition to energy dissipation, the scope of work performed at UWRL included designing the hood to provide for adequate air demand and to minimise back-splash.
The upstream end of the hood was constructed of Plexiglas in order to observe the flow in the hood, while the fixed-cone valve extends into the hood. The annulus around the valve and the upstream end of the hood was provided to allow for adequate passage of air into the hood. Air demand was measured using a circular opening in the upstream end of the hood. For the air demand tests, the annular space between the cone valve and the hood was sealed so that the air would enter through the opening and allow its volume to be accurately measured.
A fixed cone valve of 19cm throat diameter was used for the model test with a hood of 59cm internal diameter and 102cm length. Three rows of baffles 5cm tall and approximately 8cm wide, with eight baffle blocks in each row, were provided in the hood. The baffle blocks in each row were staggered to ensure that the flow through the hood was completely intercepted in three stages. A back-splash suppression ring with a 30.5cm diameter and 9cm length was installed on the inside of the hood, while the upstream end of the hood, where the cone valve entered, had a one inch annulus around the valve to enable air passage into the hood.
At the inlet of the valve, the total energy was computed from the measurement of the pressure and the velocity of flow. At the hood exit, the total energy is purely dynamic and was computed by obtaining the average velocity of the exiting jet of water.
Flow through the valve was measured using a calibrated ASME orifice plate located upstream from the valve, and the pressure at the inlet of the valve was measured with a calibrated pressure gauge.
The velocity of the jet exiting the hood was obtained by measuring the force exerted by the jet on the flat plate that intercepted the jet. A load cell was attached to the flat plate and the conservation of momentum was used to compute the average jet velocity. An alternative method was also used to obtain the velocity of the jet by measuring its trajectory and using the principles of kinematics. Results from both methods compared reasonably well.
Tests were performed with and without the baffle blocks to assess the true performance of the baffled hood as opposed to a conventional hood. It was discovered that approximately 93% of the available power is dissipated with the valve and baffled hood combination, while approximately 47% of the available power is dissipated with the valve and conventional hood combination.
Fixed cone valves require large quantities of air upstream from the conical jet. Introduction of an adequate quantity of air facilitates energy dissipation by aeration of the water jet, and helps prevent cavitation. The air demand of the hood was measured using a calibrated air velocity meter installed in an air supply pipe for the tests conducted at UWRL. The air demand was quantified and plotted as a function of the Froude Number of the jet as it exited the fixed cone valve. The Froude Number was calculated by using the theoretical velocity of the jet, divided by the square root of gravity, multiplied by the theoretical thickness of the hollow jet. The configuration of the hood was designed so that an ample supply of air was available, through the annulus, to the jet as it exited from the fixed cone valve and entered the hood.
In addition to the energy dissipating characteristics of the hood, the back-splash or return flow characteristics of the hood are considered impressive. In the past, a considerable effort has been focused on designing an optimal fixed-cone and hood angle relationship. Previous testing at UWRL has shown that the correct fixed-cone and hood angle combination will minimise the back-splash. The new hood consists of a cylinder with an upstream end cap. This cap is designed to accommodate the fixed cone valve with an annular space to allow adequate air supply in the region upstream of the jet. Another cylinder with a diameter smaller than the hood itself is attached to the end cap on the upstream end of the inside of the hood. This smaller cylinder is further equipped with a ring that deflects return flow away from the annular opening in the hood.
Such an arrangement nearly eliminated the back-splash except where a large valve opening is required for minimal upstream head. Even for this, however, the back-splash was merely a dribble. In addition to the back-splash suppression ring, the velocity of air passing through the annular space also aided in minimising back-splash.
The baffled hoods – which were constructed of ASTM 216 type 316 corrosion resistant stainless steel – were designed to withstand pulsating flow with suitable reduction in allowable stresses per existing literature for vibrations. The hooded cone valves were put into operation in the beginning of the year 2001 and are performing as intended.