In Operation & Maintenance
It is not uncommon to find system valves either not working, or if they are operational, they are not able to perform the function they have been installed for. So, what should be done?
- Aneesh Kadyan
HVAC systems are an integral part of all modern work spaces as the nature of work has changed dramatically over the last 2 – 3 decades. IT/ITES would not have been possible without the availability of large, thermally stable work environments for the thousands of software developers business processing operations that we see in the IT developments around us. Air conditioning has enabled work to be carried out 24x7, irrespective of the weather outside or the time of the day, which in turn has enhanced the productivity of the users who contribute to such developments.
A key component in any HVAC system is the system valve. Water cooled systems in particular have a large number of valves that enable the system to function effectively and as per design. In addition, Valves also allow system operators to undertake repairs to the required sections of the system without effecting the overall system, thus reducing maintenance load and increasing system availability.
While the system valves are important components of the water cooled system, they do not get as much attention as they deserve from the Operations and Maintenance teams. It is not uncommon to find system valves either not working, or if they are operational, they are not able to perform the function they have been installed for.
Overview of HVAC valves
A valve is primarily designed to regulate the flow of fluid in a piping system, including stopping, changing direction and pressure of the fluid. Valves in HVAC system can be either manual or automatic, depending on the function that the valve serves. Table 1 lists the main types of control valves used in HVAC systems.
There are two types of control valves commonly used in HVAC systems – the 2-way and 3-way control valves. 2-way control valves are used in applications where there is a variable flow while the 3-way valves are used in constant flow systems. Figure 1 shows the typical arrangement of 2- and 3-way valves.
The key characteristics of control valves are the amount of fluid flowing through the valve and the travel of the valve stem. The flow is typically expressed as a percentage of the full flow whereas the amount of travel of the stems as a percentage of travel. These two characters are plotted on a graph to give the valve characteristics curves that are used for valve selection. Figure 2 shows the types of flow characteristics. The flow across the valves is classified into:
• Linear flow where the flow is directly proportional to the amount of opening of the valve or the travel of the stem.
• Quick opening valves allow a higher amount of flow at the initial travel of the stem, with the flow plateauing towards the full length of travel. Butterfly valves are quick opening and are used in applications requiring quick shut off.
• Equal Percentage Valves – In these valves, equal percentage of valve travel will result in an equal increase or decrease of flow. Ball and globe valves are equal percentage valves.
Flow control valves are specified based on the following parameters:
• Amount of flow across the valve and the pressure drop experienced across valve. The flow coefficient is used to link these two parameters.
• Maximum pressure that the valve can withstand when closed.
• Pressure drop across the valve in the system.
• Parameters of the fluid – Temperature and pressure – that the valve can withstand.
Figure 1: 2- and 3- way valve arrangements...
Figure 2: Valve flow characteristics...
HVAC system valve maintenance
The design of modern valves is quite robust, and hence they are highly reliable and rugged. When a system is commissioned, due to the valves deign, there are very limited breakdowns and difficulties in operations. This invariably leads to a lower emphasis on maintenance of the system valves by the O&M team. In commercial and retail buildings, since the operations are nearly 24x7, the valves are typically left in the open conviction and not operated in the normal course of operations. But when there is a defect in the system, and a section has to be isolated – does the valve get operated? Invariably, at this time, due to the valve not having been operated and maintained effectively, the valve does not function and the valve manufacturer is blamed for giving a low quality component.
Maintenance of valves is relatively easy but is still not undertaken. The cost to repair a valve is up to 30% of the cost of a new valve. Replacement valves are also not easily available as the valves can sometimes be custom built.
In addition, spare valves cannot be stored for all the different valves that are used in the HVAC system of a building. Thus, an effective maintenance plan for valves will be essential to keep the stem functioning at full capacity. The main maintenance activities that are required for HVAC system valves are:
• Visual inspection for any lose bolts, flanges
• Greasing of drive mechanisms
• Listening for abnormal noise, which is an indication of fault inside the valve casing
• Six monthly testing of the valve for leakage should be undertaken.
• Valves that are not operated frequently should be operated through the full travel at least once a month.
• Where pressure gauges are provided at the upstream and downstream of a valve, the pressures should be compared to the valve manual to detect any wear in the valve body.
• Annual shut down of HVAC systems should include a check of large valves as the shutdown gives an opportunity to carry out the repairs, which would not be possible during the system operation.
Valves operating at the design parameters in an HVAC system are as critical for the system to work efficiently. A defective valve will lead to higher losses in the system, which in turn will result in higher pumping power and cost.
The cost and effort to maintain control valves is very minimal but is often neglected by the O&M team. A simple valve maintenance program, with a focus on basic maintenance practices is all that is needed to keep the valves in a healthy state.
The impact of a defective valve is unfortunately only felt when the valve is required to close – and it does not! The O&M team should thus include valves’ maintenance as part of their planned preventive maintenance schedule, which will minimise the potential for system failure as well as lower maintenance costs.
Director - Operations
CBRE South Asia Pvt Ltd.
Asset Services - India
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