Eric Dobbs, Severe Service Manager, Justin Goodwin, Director of the Steam Conditioning Group, and Mark Petruccelli, Critical Service Sizing and Selection Specialist recently published an article in the April 2020 issue of Process Heating magazine describing the desuperheating and steam conditioning processes, and detailing the critical equipment and installation requirements for a successful application. Their article is titled The Basics of Desuperheating and Steam Conditioning and is summarized below:
Desuperheating versus Steam Conditioning
Desuperheat systems add water to process steam to lower its temperature and approach saturation. Steam conditioning systems provide desuperheating and also reduce steam pressure. The authors describe why these processes are required:
Two primary uses for steam in the power and process industries are performing mechanical work and as a heat transfer medium. Superheated steam is preferred for mechanical work since it does not contain entrained water droplets which can damage rotating equipment. For heat transfer applications, steam at or near saturation provides more efficient heat transfer than superheated steam.
Some plants also have a series of fixed pressure steam headers and steam must be let down from one header to another to balance process flows and maintain pressure. These let down stations often create superheated steam. In this application, a steam conditioning system is a logical choice as it provides the required pressure reduction and controls steam superheat.
How Steam Conditioning Works
The authors describe a typical steam conditioning system (Figure 1).
Figure 1: The main components of a desuperheating and steam-conditioning system.
During operation, steam is fed into a pressure reducing valve, and this steam flow enters the desuperheater section. There, it receives fluid from a spray water valve to cool it. Temperature and pressure readings are taken downstream of the desuperheater section. These readings are used as inputs to the control system, which regulates the actions of both the pressure-reducing valve and the spray water-valve as needed to maintain the desired temperature and pressure.
The spray-water valve (not shown) injects water just downstream from the pressure-reducing valve and must provide Class V shutoff to avoid leaking water during no/low flow conditions. This valve usually has anti-cavitation trim due to the high pressure drop experienced between its inlet and outlet.
Design Challenges
If installed incorrectly, a desuperheat system can cause significant erosion damage to the downstream piping as the liquid strikes the walls of the pipe. The authors explain:
The main challenge in desuperheating is the creation of temporary two-phase flow in the steam pipe through the introduction of spray water. Proper design of a desuperheater aims to limit any negative effects by reducing the amount of time the water resides in the system.
High steam velocities help break up and evaporate the water droplets, but low steam velocities don’t provide this functionality and should therefore be avoided. If unevaporated water reaches a temperature measurement element, the saturation temperature of the water will be read rather than the actual steam temperature. For this reason, the desuperheat temperature should always be at least 10 degrees above saturation.
Installation Considerations
There are a number of key design aspects that must be considered when designing a desuperheat system. They include:
Straight pipe length: The straight pipe downstream of the water injection point must be long enough to ensure the water droplets can navigate a pipe bend and not strike the wall.
Temperature sensor distance: The desuperheat temperature sensor must be far enough away to ensure all the water is fully evaporated.
Pipe line size and schedule: The size of the pipe will often dictate the style of water injection, the type of nozzle, the straight pipe length, and the temperature sensor distance.
Water injection system design: Every spray water system should have a strainer installed upstream with a mesh size appropriate for the nozzle. Clogged nozzles (Figure 2) will direct water against the pipe walls and greatly reduce the rate of evaporation.
Figure 2: A functional spray nozzle (left) generates a smooth, flat pattern of water easily sheared by steam flow. A clogged nozzle (right) will direct water against the pipe walls and damage them.
Injection system type: If the pipes are smaller (1” to 8”), a venturi-type nozzle is a good choice. Larger desuperheat systems (8” to 60”) will usually employ a ring-style injector.
System drain design: Desuperheaters system should be designed with adequate drains. Each drain should be automated to remove any water that fails to evaporate or condenses during shutdown.
Steam Conditioning Systems
Historically users have installed desuperheaters separate from the pressure reducing valve, but there are benefits to buying the steam conditioning system as a unit. The combined solution reduces installation costs, and locating the desuperheater just downstream of the pressure valve makes it more efficient since the water droplets are more easily evaporated in the turbulent flow exiting the pressure valve.
Final Considerations
The authors summarized their article as follows:
Desuperheating and steam conditioning systems are widely used in the power and process industries. Control valves are key components of both of these systems, and there are many variables affecting their operation. An experienced valve vendor can examine all operating conditions and help ensure the appropriate valves are selected to provide efficient operation with minimal required maintenance.
Figures all courtesy of Emerson
About the Authors
Eric Dobbs is the Severe Service Manager in Fisher’s global industry sales group. He has held positions in product, design, and sales engineering, with over 6 years spent specifically on steam conditioning. Eric holds a BS degree in Mechanical Engineering from South Dakota State University.
Justin Goodwin is the Director of the Steam Conditioning Group at Fisher. He is responsible for the design and technical support of steam conditioning and desuperheating equipment, and provides direction, technical oversight, and training for Emerson’s global steam conditioning business. Justin holds a BS degree Mechanical Engineering from Iowa State University
Mark Petruccelli is a Sizing and Selection Specialist—Critical Service at Fisher, with 25 years’ experience in steam conditioning and severe service applications. Marks holds a BS degree in Chemical Engineering from The Pennsylvania State University.