Improving a Steam Conditioning Application Saves Money for Oil, Gas, & Petrochemical Facilities

since they require a reliable demand for the heat produced, CHP (combined heat and power) systems are ideally suited on sites where there is an adjacent chemical works or industrial complex that runs 24 hours a day. In return, the primary demand for many industries such as pulp and paper, refineries, sugar and petrochemicals facilities require steam very close to a saturation temperature.

Normally a CHP plant is an installation where there is a simultaneous generation of useable heat (usually steam and sometimes hot water) and power (usually electricity) in a single process. If the steam is supplied at a temperature that is too high, then in certain circumstances the product or equipment is damaged. If the temperature is too low there will be excess water, control will be lost, and there may be damage to piping and downstream equipment owing to the excess of water.

The problem is that the bypass valves used in this application have a limited desuperheating turndown, since a certain minimum steam velocity downstream the valve is required to get a mixing effect with cooling water. Usually, 8 m/s is the required minimum steam velocity. Maximum velocity is seldom above 55 m/s in such applications, giving maximum turndowns of 8/55 ˜ 1:7. It is therefore necessary to bypass a certain minimum flow through each bypass valve when the process demands are higher than what the turbine can pass. A calculation example is shown below.
Calculation examples of minimum flows:

Pos. 1A ˜ 5 t/h
Pos. 1B ˜ 6 t/h
Pos. 1C ˜ 10 t/h

The consequence of this (for example) is if the demand on the 4 bar line is 50 t/h, the bypass valve must pass 10 t/h and the flow through the turbine must be decreased to 40 t/h. The electrical production must therefore be decreased, by 9 t/h, which means the electrical output will decrease with approximately 1.8 MW. 5 and 10% of the possible electrical production on a yearly basis is lost because of this. Based on data from a typical paper mill with 300 operating days a year, this would mean large savings if the limited turndown could have been solved. If we assume the difference between production cost of steam and the price of electric power to be 2 cents/kWh (˜ 20 US$/MWh) and the loss is 5%, the income shortfall based on a yearly possible electrical production of 144,000 MWh will be 144,000 x 5% x 20 US$/MWh = 144,000 US$/year.

To present a reliable and high performance solution for a bypass process steam conditioning valve, CCI Sweden has developed the VST-SE. The VST range of valves is primarily used in industrial plants such as a CHP for the conditioning of auxiliary and process steam.
The VST-SE meets the operational demands of the industry by providing the following features:

• Resistance to thermal shock and fatigue – the VST-SE has a fully forged machined valve body both outside and inside to handle thermal fatigue which is critical for reliable service.
• System integrity – a pressure seal bonnet to maintain tightness regardless of thermal transients. This allows for quick and easy in-line access to valve internals for inspection and service.
• High turndown – with steam atomization the VST-SE will achieve turndown with respect to desuperheated steam flow greater than 50:1. By providing very high turndown with regard to steam flow and temperature, this will maximize the steam flow and temperature and therefore MW from the steam turbine.

The amount of process steam to textile mills and chemical operations is supplied according to specific requirements. By providing valves such as the VST-SE to accompany these systems, performance is improved while lowering costs over the short and long-term, offering a functional, reliable solution to complex challenges.

Published in Solutions Winter 2002