Steam Flow Meters More

The following article was written by Solartron Mobrey Ltd. and available on their web site linked below.

Ensuring energy efficiency through effective monitoring of steam.

The steam boiler house is an area where significant financial savings can be made through energy efficiency. Fiscal monitoring of energy used is vital for optimisation of processes and planning investment in machinery. Despite this the boiler house rarely attracts the interest from management, and therefore the investment, that it deserves.

The majority of boiler houses do monitor the flow of steam from the boiler house to process, but often the accuracy of the measurement system is questionable. In such a harsh environment, most traditional flow measurement instrumentation suffers from wear, corrosion and fatigue. If it is has been in place for a long time without proper maintenance and/ or has not been calibrated for a long time, then measurement errors are significant. For example, without maintenance, a traditional orifice plate is likely to be eroded by steam flow to a point the reading has no relevant meaning within a few years, or even months.

Measuring the mass flow of saturated steam, which at the measuring point is invariably wet, is not easy. The selection of the measurement element is critical to ensure long-term accuracy. This article provides a quick reference guide to the wide range of technologies available and is designed to help those making the selection find the right solution for their budget and application.

Steam measurement technology

Steam flow is a harsh environment and the presence of water droplets and other impurities flowing at high velocity precludes the use of some types of flow meters. Flow measurement technologies that are useable in the steam environment are: Orifice plate, averaging pitot, shedding vortex, variable area, venturi, nozzle and (to a lesser extent) corriolis.

Orifice plate, variable area, venturi and nozzles type meters fall into the same category and all rely on the pressure drop caused by the increase in velocity as the fluid passes through a restriction. The differential between upstream and down stream pressures is proportional to the flow velocity.

The averaging pitot tube is positioned across the pipe diameter to measure the impact and suction pressure of the fluid. This also induces a differential pressure, which is proportional to the flow velocity, but it does so without affecting the velocity of the flow.

The shedding vortex meter has a similar profile to the averaging pitot tube but it relies on the oscillating vortex eddies shed by a non-streamlined body positioned in the flow. The number of eddies is proportional to the flow velocity.

The Corriolis meter uses a straight or bent tube, which is forced to vibrate at its natural frequency. As the fluid to be measured flows through the tube it accelerates causing the tube to twist. This twisting motion is proportional to the flow velocity.

Selecting a steam meter

The purpose of a steam flow system is to move energy to where it is to be used. It follows, therefore, that the introduction of a restriction into the pipe as part of the metering equipment reduces the efficiency of the system.

This basic principle brings into question the use of the traditional orifice plate device, and particularly variable area meters, which introduce an unrecoverable pressure drop into the system. The consequent energy cost can be substantial; this type of device is energy inefficient by today’s standards.

In contrast, shedding vortex meters and averaging pitot tubes create negligible line loss – less than 70mbar  (one inch of water) on an equivalent 10 bar system (i.e. is less than 0.7%).

Fixed and variable orifice plates

Orifice plates are prone to wear. In the harsh, wet steam environment, the sharp-edge of the hole in the plate is soon eroded. Without frequent servicing and regular replacement, the orifice plate quickly looses its accuracy.

Variable devices also suffer mechanical wear, and some components may be affected by temperature change. Although compensation for these components can be built-in, long term accuracy is compromised.

Do not be misled by the figures that manufacturers of variable orifice plates quote for ‘turndown’. (e.g. 100:1) The limitations of the steam minimum and maximum flow, and the effect of actual site conditions rarely allow this figure to be achieved. Calculate the maximum expected flow and the minimum achievable flow for the meter; this will give the actual available turndown for the application which has nothing to do with the figure quoted for the meter.

Shedding vortex

Vortex shedding meters present a small profile to the flow and therefore the pressure drop is negligible making them energy efficient. These devices give a practical turndown of 30:1 although they do not tolerate low flow rates so this turndown may not be realised on all applications. Vortex meters are susceptible to error from the effect of mechanical noise. The units exhibit long-term accuracy although wear can take place making the vortex less defined, but this is not a significant problem.

Averaging pitot tube

Unlike other flowmeters, installation of an averaging pitot tube is straightforward, requiring minimum downtime. The only incursion into the pipe is a one-inch weldolet into which the pitot is screwed. This and the fact that it introduces a negligible pressure drop into the system make it highly energy efficient and a popular choice for a many common steam measurement applications.

When selecting a pitot tube, remember that shape is important; only those with a square cross-section retain accuracy over the full measurement range. The accuracy of those with round bars is affected by changes in flow.

The averaging pitot tube has no minimum flow velocity; it measures even very low flows accurately, and has a turndown of up to 100:1. The critical factor in a low velocity steam flow measurement is the capability of associated pressure transmitter. The ‘turndown’ of the combined system is usually quoted at about l0:1.but this is easily expandable to 100:1 by using two differential pressure cells.

Corriolis

Corriolis is the most accurate of all the measurement technologies under discussion. It has a very wide measurement range, but it is expensive. Where very high accuracy is paramount, regardless of cost, this is the best choice.

A steam flow meter is a significant investment, which can provide essential long-term plant and process information. Choosing a flowmeter is a minefield for the uninitiated and it is quite easy to be misguided by the wealth of data put out by manufacturers.

Turndown ratio is often seen as the major selection criteria, however the turndown specified by the manufacturer is for maximum and minimum flow conditions for the instrument not the application. In practical terms advertised turndown is seldom realised because the maximum flow at site is to low. Furthermore flow meters can be used on liquids and gas and the advertised criteria covers all applications which and may not be applicable to steam use.

Long-term accuracy can only be guaranteed by a measurement device that is not prone to wear. Orifice plates and variable area devices loose their accuracy over time, particularly in a dirty, harsh environment like steam.

Averaging pitot tubes and shedding vortex devices have fixed characteristics, ensuring accurate measurement over a long period of time.

For more information, please contact:

Website: www.solartronmobrey.com

September 2004

Source: Solartron Mobrey Ltd, web site 3/2005;