SCADA for Surge Control

By Ahmed Hafaifa, Attia Daoudi and Mouloud Guemana

The most efficient operation of gas and oil installations is subject to many technical and economic constraints. Certain installations cannot stop functioning without incurring enormous financial losses, but technical defects can also harm the quality of their products. The SCADA system can prioritize necessary maintenance activities to prevent inopportune breakdowns, reduce the time fixed assets are under maintenance and optimize their operational life though just-in-time intervention. SCADA systems have successfully addressed numerous industry problems [1], [13], [15], [18] and [19]. In this article, we describe how SCADA systems could be applied to maintenance challenges in facilities of the Algerian oil company Sonatrach.

To illustrate the efficiency of the proposed supervisory schema for the process control, the SCADA system has been constructed as a centralized system using a proprietary control computer and operating system. The use of a SCADA system in a gas compression plant facilitates planning, assigning, supervising and participating in the operation, maintenance and repair of the system, and related work. The disadvantage of this centralized system is that when a facility wants to improve or add some functions, it sometimes has to upgrade the existing computer’s memory capacity or replace it with a higher-grade computer.

On the other hand, a recently emerging trend in the development of computer and communication technology has been making it possible to establish open, distributed computer systems. Such technology is also being introduced into SCADA systems in gas compression plants. In this article, we give the experimental results of the use of the SCADA system in a gas compression system for supervision and control and data acquisition. We found that the high reliability, functionality, performance and high-level human interface add to the expandability and flexibility in configuration.

This paper describes tools and techniques that can and have been used to model transient flows and performance, mechanical and control responses, and time-dependent head in compressor systems. The tools used by SCADA control include finite time step programs that simulate control systems, valve actuators and the opening (or closing) rate of valves, with the resulting flows. The volumes and lengths of station piping, scrubbers and coolers, including temperature effects are accounted for. SCADA supervisory control models also track the performance of centrifugal compressors at different speeds, account for the rotation inertia of compressor trains, and evaluate the thermo physical properties of gas streams.

Industrial compression system

Compression systems are used, for example, as part of a gas turbine for jet and marine propulsion or power generation, in superchargers and turbochargers for internal combustion engines, and in a wide variety of industrial processes [4]. In this paper we focus on centrifugal compressors that are used in natural gas pipeline transportation. This type of centrifugal compressor can exhibit a variety of instabilities under different operating conditions. Moore and Greitzer in [20] developed a phenomenological model for rotating stall and surge. This pioneering work modelled the compression system with just three components:

1. The first component is the inlet duct that allows infinitesimally small disturbances at the duct entrance to grow until they reach an appreciable magnitude at the compressor face.
2. The second component is the compressor itself, modelled as an actuator disk, which raises the pressure ratio by doing work on the fluid.
3. The third component is the plenum chamber (or diffuser) downstream, which acts as a large reservoir and responds to fluctuations in mass flow with fluctuations in pressure behind the actuator disk.
   In this paper, we are considering a compression system consisting of a centrifugal compressor, valve, compressor duct, plenum volume and a throttle (Figure 1). The throttle can be regarded as a simplified model of a turbine [7], [8], [10]. The system is shown in Figures 1, 2 and 3.

Figure 1: Centrifugal compressor

Figure 2. Anti surge valve

Figure 3: Centrifugal compressor

The model to be used for controller design is in the form:

Where 
p_p = the plenum pressure
K = a numerical constant 
p₀₁ = the ambient pressure
ρ₀₁ = the inlet stagnation density
V_P= the plenum volume
m = the compressor mass flow
k_t = a parameter proportional to throttle opening
A_l = the area of the impeller eye (used as reference area)
L_c = the length of compressor and duct
ηi = the isentropic efficiency
N = the spool moment of inertia
Δ_{h ideal} = the total specific enthalpy delivered to fluid
c_p = the specific heat capacity at constant pressure,
c_v = the specific heat capacity at constant volume
T₀₁ = the inlet stagnation temperature
k = is the ratio of specific heat, 

The present work has analytically integrated the right hand side of equation (1), presented by three ordinary differential equations of the Moore and Greitzer model that give rise to modelling the compression system: the first for the non-dimensional total-to-static pressure rise   across the compression system; the second for the amplitude of mass flow rate fluctuations m; and the third for the non-dimensional, spool moment of inertia [20]. The two first equations of (1) are equivalent to the model of [4]. Note that the Moore-Greitzer model does not attempt to explain what physical mechanism triggers these instabilities.