MAZDA'S ADVANCED CONCEPTS IN
DESUPERHEATING TECHNOLOGY

Selection Considerations

When selecting a desuperheater for an application, “turndown” is an important consideration. Turndown is the ratio of maximum to minimum conditions: maximum steam flow divided by minimum steam flow, or maximum water flow divided by minimum water flow.

Simple applications often call for one design point and no turndown, and require the simplest and least expensive models; typically a single venturi desuperheater.

High water turndown is easy to achieve with a separate water control valve on a venturi model, or an appropriate water nozzle arrangement on a multi-nozzle type. High steam turndown is not so simple, since efficient desuperheating can only be achieved with adequate steam velocity. However, pressure drop becomes a critical factor as steam velocity increases. Extreme steam turndown presents a design challenge even forheteasoned engineering staff.

Statement of Problem

The installation in question is located in one of the largest petrochemical plants in the Far East. Mazda was faced with the following critical process conditions:

System A

Effectively, the spray water requirement varied between 165 kg/hr and 28764 kg/hr, or a water turndown of 174:1.

System B was similarly complex, with steam turndown requirement of 47:1 and water turndown of 50:1.

Together with close-to-saturation final temperature requirements, the high turndown and variable process conditions made for an extremely difficult desuperheating problem.

Analysis

High water turndown is no problem for our Multi-Nozzle units, which are designed to handle up to 300:1 water turndown. However steam turndowns of 21:1 and 47:1 were another matter.

Steam velocity plays a critical role in desuperheating. If the steam velocity drops below 8-10 m/s (26-33 ft/s), desuperheating will be incomplete and a continuous amount of unevaporated spray water will be carried downstream. This is detrimental to equipment downstream in terms of thermal shock and pipe impingement by water hammer, and causes a lesser-known problem that will be illustrated by the following example.
If the required spray water quantity
for a particular installation is 1000 kg/hr, engineers will generally size desuperheaters with a safety factor, of perhaps 20%. If the steam velocity is not high enough however, desuperheating will be incomplete and a percentage of the spray water will not evaporate. As a result, the downstream temperature will exceed design parameters and the control system will compensate by opening the water control valve to allow more spray water through (say 1150 kg/hr). The excess water will be present no problem due to the safety designed into the unit and, with the temperature dropping to the design point, it will appear that the system is operating properly. In fact the system will be consuming15% more water than is necessary. The excess water will drain away (assuming the drainage is properly designed), and will repretsent a considerable loss when the cost per kg of treated plant water is taken into account.

Solutions

Mazda’ desuperheater arrangements are shown in sketches A and B In the next page.