On-Line Energy Monitoring In A Control System

On-line Energy Monitoring in a Control System

Terry Blevins Mei Yang

Principal Technologist Intermediate Software Engineer

DeltaV Product Engineering DeltaV Product Engineering

Emerson Process Management Emerson Process Management

Austin, Texas 78759 Austin, Texas 78759

KEYWORDS

AGA, Gas Flow, Steam and Water Properties, Energy Metering

ABSTRACT

An important component in improving process performance and plant profitability is the exact measurement of purchased energy and the utilization of energy within the process. Such information should be available in real time to gain maximum benefit from this information in day-to-day operations. An accurate gas measurement based on the AGA or ISO standards is often the basis for accurate account of purchased energy. Also, the enthalpies to an accuracy of 0.1% are needed for steam and water at process operating pressures and temperatures to perform energy balances and evaluate the efficiency of unit operations. The effort and time required to engineer and implement such online calculations may be significantly reduced if gas, steam, and water property algorithms are embedded within the DCS or control system itself.

In this paper, an example is provided of how these calculations have been implemented as function blocks in a modern, scalable process control system. Details of the nine function blocks for gas, steam, and water properties are provided with examples of how these may be used to address typical energy monitoring and calculation applications.

INTRODUCTION

The cost of purchased fuel and the efficient utilization of energy are playing an increasingly important role in today’s plant economics. The sudden rise in natural gas prices in the last two years has sparked renewed interest in accurately metering any purchased fuel and determining where energy is being used within the process. This interest is based on the realization that the first step in reducing energy consumption is to have an accurate account of how much energy is being used and how it is being used. Based on this knowledge, it is often easy to identify areas that would most benefit from improved control or modifications in the process to improve efficiency.

The measurement, installation requirements, and calculations for accurate metering of natural gas flow and energy in a given world area are addressed by various standards organizations. The AGA (American Gas Association) standards are predominant in North America while the ISO (International Standard Organization) requirements tend to prevail elsewhere, although there are many exceptions. The International Association for Properties of Water and Steam (IAPWS) sets the international standard for water and steam properties.

An accurate measurement of gas or determination of steam and water properties are complicated by the fact that the calculations that must be done to precisely account for non-linear changes as a function of the operating conditions and flow rate. The general flow and steam property equations published by these standard organizations are relatively complex and various approaches may be taken to implement energy accounting systems that utilize these calculations. In some cases, digital flow computers or PC resident software applications are available that incorporate these calculations. However, it is often a challenge to integrate such software or devices into a process control system and plant information system. Many of these problems are can be resolved if these calculations are embedded within the control system.

BACKGROUND OF ENERGY METERING

The accurate measurement of gas flow in a pipeline is of significant concern to energy companies involved in the custody transfer of natural gas. Also, the accurate metering of steam generated and used in manufacturing is of interest to many companies. To meet these needs, specialized devices, special devices, known as multi-function flow computers, have been developed for a variety of flow meter types in liquid, gas, and steam applications. Many of these flow computers were designed as a remote terminal unit (RTU) for operation in locations far from a plant site. For example, remote operation is often required for flow measurement for underground natural gas storage, metering and shutdown stations for natural gas transmission and supply networks.

In most cases, these devices are fully programmable and may be used to address a wide variety of flow applications – volume, mass and heat flow rates for steam most liquids and hydrocarbon gases. In most cases these calculations are based on published standards. The most common are:

Hydrocarbon Gas Flow

AGA3 (1985, 1992)

ISO9951 and AGA7 (1996) – turbine meters

AGA8 (1985, 1994)

NX19

Steam and Water Properties

IFC-67 and IFC-97

The user may have the option of selecting the measurement and calculation units i.e. SI or English.

Measurements for these flow and property calculations are normally accessed as galvanic isolated analog inputs (4-20 ma.). Also, inputs may be single- or dual-pulse train from a turbine meter. These devices may also support HART transmitters and analyzers. This allows direct management of pressure, temperature, and differential pressure transducers via the HART protocol. For on-line input of gas analysis, a serial interface for an on-line chromatograph or a calorimeter may be supported. Also, manual entry of data from the laboratory is often supported.

In some cases, the calculated value may be provided as an isolated analog (4-20ma) outputs that can be chosen to follow the volume flow, mass flow, etc. Normally provisions are made for remote host access through a modem, direct RS232/485/422 connection, VHF or Spread Spectrum radio communications, etc.

Since the remote host may not be always connected, local accumulation of flow must be done and special provisions made to insure that information is not lost on a power failure e.g. with special battery or internal flash memory.

Figure 1. Flow Computer for Gas, Steam, and Water Applications

For many SCADA applications, the multi-function block computer is a low cost and effective solution. However, in a process plant the measurements associated with gas, steam, and water are normally wired into the control system. In many new installations these may be fieldbus measurements. Thus, from an engineering, installation, and maintenance perspective, it is highly desirable to integrate the gas flow and steam and water properties calculation into the control system rather than purchase and interface RTU’s to the control system. However, many traditional process control systems do not provide support for the IFC steam properties or AGA/IOS gas flow calculations. Thus, if these calculations are needed for energy accounting in a process plant, then extra software may need to be integrated into the process control system.

To assist manufacturers in implementation, these standards committees within AGA, ISO, and IFC have provided example code and tables of expected results. Example source code for these calculations may be purchased. Also, numerous companies offer MS Windows applications for AGA flow, and steam and water properties. These applications are often marketed for use in power plants such as steam turbines, pumps, boilers and heat exchangers. However, limited support may be provided in the integration of this capability into a process control system. For example, the function provided for steam and water properties may be included in the systems as Microsoft Excel Add-in. Tighter integration may be required to automatically access measurements and view results within the control system. For example, this might involve linking a dynamic link library (DLL) with C, C++, or Visual Basic. The cost of the basic software for AGA flow calculations or steam and water properties is often very low. However, the cost to integrate the software, engineer applications, provide training on its use, and maintain the software can often be many times the original cost of the software. Also, if the software is integrated into a PC as shown below, the percent uptime of the application may be much lower than that of the control system because of the lack of redundancy, other applications that may impact PC operation.

Figure 2. Traditional Integration of Gas, Steam, and Water Calculations

The problems of integration can be eliminated if these gas, steam, and water calculations are standard in the control system.

EMBEDDING GAS, STEAM, and WATER CALCULATIONS

The problems of integrating applications into the control system may be avoided if these calculations are a standard feature of the process control system. Also, greater reliability may be provided since many control systems support redundant controllers and I/O. The approach that a manufacturer takes in integrating gas, steam, and water calculations into the control system will depend greatly on the system architecture. Many modern control systems allow measurement, calculations and control to be implemented using a standard set of function blocks. For this type of system architecture, it is logical to structure the gas, steam and water calculation blocks as function block. This allows the same graphical engineering environment using simple drag and drop techniques to be applied in implementing energy calculations.

An important part of Fieldbus Foundation specification on which many fieldbus devices are based is the function block architecture for monitoring, calculation and control. All calculations are done in engineering units and represented in floating point. Also, function block inputs and outputs by definition must support both a value and status. The status attribute explicitly indicates the quality of the measurement (i.e. Good, Bad and Uncertain) and whether the value is limited. Input status is used by function blocks in their calculations and propagates status to the block outputs. Thus, when a control system is designed to utilize this fieldbus architecture, then the blocks for gas flow and steam and water properties may use status as part of the block algorithm.

There are three basic forms of the AGA flow calculation that may be used depending on the measurement requirement and the inputs that are available for the calculation. To simplify the definition of AGA flow, it is possible to structure one function block support all three version of the calculation, as shown in Figure 3.

TEMP_IN

Figure 3. Support of AGA Flow with One Function Block

Parameters are provided in the block definitions that allow the user to select the form of the calculation. A block such as the AGA flow calculation requires many configured parameters that can be structured as “contained” parameters. This allows these parameters to be accessed and viewed for configuration during the execution of the function block and also during on-line operation.

IMPLEMENTATION EXAMPLE

In a specific case the AGA flow calculations and steam and water properties are implemented as a function block. These blocks are designed and executed in the controller of the control system. Therefore these calculations run on the same scan period as other function blocks that make up a control module. This allows values such as specific volume of steam to be use to compensate flows that are used in control as well as monitoring applications. The quality of inputs to the block is directly indicated by the status attribute. Thus, the blocks are designed to automatically use status in the block algorithm to determine the status of block outputs. The following blocks are provided to address energy calculations.

AGA Flow (AGA) - The AGA function block implements the American Gas Association flow calculations for natural gases, namely, AGA-3 (American Gas Association, Report No.3), AGA-7 and AGA-8. The AGA-3 standard is applicable to virtually all single phase, Newtonian fluids under turbulent flow. The AGA-7 standard is applicable to all fluids and gas. The AGA-8 standard is used to calculate the density and compressibility of natural gases and other related hydrocarbon gases given composition, temperature and pressure. Figure 4 is example of using the AGA block.

Figure 4. An Example of Using AGA block

The block can be used to calculate the instantaneous flow rate (mass and volumetric) and accumulation with a differential pressure meter or turbine meter. It can calculate accumulation for every day, or from a specific time. The user can choose to have it to calculate the density and compressibility if he knows the gas composition, or assign the density and compressibility. The function block supports two sets of units: US (United States) engineering units, also known as English, Imperial, and Standard, and SI (System International) engineering units.

Isentropic Expansion (ISE) - The ISE function block calculates the final enthalpy for isentropic expansion of steam to a given pressure for given entropy. If the entropy-pressure point lies in the two-phase region, the combined enthalpy of steam and water for constant entropy is provided. Steam quality (percent moister) and steam temperature are also calculated. The isentropic operation is valid for both the single and two-phase region.

Steam Density Ratio (SDR) - The SDR function block calculates the square root of the ratio of steam density to the density of steam corresponding to a flow meter calibration pressure and temperature.

Saturated Temperature (SST) - The SST function block calculates the steam enthalpy, entropy, specific volume and pressure for saturation conditions specified by a given temperature. It applies saturated steam.

Steam Properties (STM) - The STM function block calculates steam entropy, entropy and specific volume for a given gauge pressure and given temperature. It applies to superheated steam and saturated steam.

Saturated Steam Property (TSS) - The TSS function block calculates the steam temperature at saturation given steam pressure. It applies to saturated steam.

Water Enthalpy (WTH) - The WTH function block calculates enthalpy of water for a specified temperature.

Water Entropy (WTS) - The WTS function block calculates entropy of water for a specified temperature.

These energy and metering function blocks may be used with other measurement blocks to calculate the energy content of a stream, compensate a steam flow measurement for specific volume, etc. One example of how these blocks may be used is shown below. In this example, a steam flow measurement is being density compensated to correct for the specific volume being different that the calibration condition for the orifice.

Figure 5. An Example of using STM and SDR block

REFERENCES

1. ASME International Steam Tables for Industrial Use, ISBN 0-7918-01543, ASME Press, New York, N.Y.
2. Compressibility Factors of Natural Gas and Other Related Hydrocarbon Gases, AGA Report 8, 1992, Catalog Number XQ9212, American Gas Association, Arlington, Virginia
3. Orifice Metering of Natural Gas and other Related Hydrocarbon Fluids, AGA Report 3, 1990 Catalog Number XQ9017, American Gas Association, Arlington, Virginia
4. Orifice Metering of Natural Gas and other Related Hydrocarbon Fluids, AGA Report 3, 1995 Catalog Number XQ9211, American Gas Association, Arlington, Virginia
5. “Scalable Control System: easy-to-use MPC implementation platform,” http://easydeltav.com/eventcentral/videos/casestudies/