Editor’s choice
AT A GLANCE COMPARISON OF STA AND SLA LOGIC SOLVERS
The STA can handle only one SIF at a time, while the SLA can handle up to three SIFs.
The SLA has easy-to-use voting logic capability, the STA does not have any voting capability.
Both the STA and SLA are exida-approved and SIL 2/3 capable.
Figure 5. Alarm four configuration screen showing how the SLA
Faults are assigned to an internal alarm, in this case to ALM4.
straightforward screens for device configuration. To demonstrate the SLA’s ease of configuration for this application example, we have provided a few PACTware screenshot examples below. To configure the SLA as a logic solver for three separators, PACTware is used to create the high- pressure alarms and voting logic required to initiate safety shutdown. Outlined below are the key configuration requirements and associated PACTware screenshots demonstrating the SLA’s ease of programming and setup.
SLA KEY CONFIGURATION REQUIREMENTS FOR HIGH- PRESSURE SIF
Required separator alarms: Separator 1
• Create ALM1 = Hi Pressure Alarm Separator 2
• Create ALM2 = Hi Pressure Alarm Separator 3
• Create ALM3 = Hi Pressure Alarm SLA Fault Alarm
• Create ALM4 = Set Alarm Type as Discrete, 1oo1 Voting, Source 1 is RO4 (SLA Fault Alarm)
Required separator shutdown outputs:
Separator 1 • Create RO1 = 1oo2 voting on ALM1
(High Pressure) or ALM4 (SLA Fault Relay) Separator 2
• Create RO2 = 1oo2 voting on ALM2
(High Pressure) or ALM4 (SLA Fault Relay) Separator 3
• Create RO3 = 1oo2 voting on ALM3 (High Pressure) or ALM4 (SLA Fault Relay)
The SLA has an onboard event logger that captures key data that can be exported to a .csv file, while the STA has no logging capabilities.
The SLA supports read-only MODBUS/TCP and MODBUS RTU industrial protocols.
The SLA has an intuitive
equation/expression editor for math and logic functions that is easy to use, while the STA has no math or logic capability.
Both SLA and STA support operating temperature ranges of -40 to 85°C.
CONCLUSION
Neglecting to monitor separator pressure levels can result in serious consequences, such as damage to the separators’ structure, environmental contamination, and risks to those nearby. High
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September 2024 Instrumentation Monthly
The SLA can handle up to 16 alarms while the STA can only handle two alarms.
The SLA includes an onboard read-only web server that allows all SLA parameters to be read with an off-the-shelf web browser. The STA includes no digital interface or output.
The SLA accepts six analogue inputs, while the STA accepts only one input.
The SLA has four discrete input/output channels that could be used to suppress the shutdown circuits/alarms during process startup. The STA has no discrete input channels.
The SLA has three analogue outputs which can be used to retransmit the separator’s pressure readings to an auxiliary monitoring device, while the STA has only one analogue output.
The SLA analogue outputs pass HART data when corresponding analogue input channels are connected to HART devices allowing full access to connected field devices by HART hosts. The STA does not pass any HART data.
Figure 6. Relay output configuration screen demonstrating how relay outputs are assigned to each of the separator shutdown SIFs. This is where the simple 1oo2 voting logic is implemented to trigger a separator shutdown if either a high-pressure alarm exists or there is a fault detected with the SLA logic solver.
pressure within the separators can cause leaks, ruptures, or even catastrophic explosions, which can have severe impacts on both operational efficiency and safety protocols. Therefore, it is essential to implement strong pressure protection measures, such as a Safety Instrumented System, to mitigate potential hazards and maintain optimal performance of the separators. Logic solvers are critical components for ensuring the safety of separators, especially when it comes to Safety Instrumented Functions. While traditional single-loop logic solvers like the STA are successfully used, newer solutions like the SLA multiloop logic solver offer more advanced capabilities, particularly in applications that require monitoring of multiple separators.
The SLA is a logic solver that offers numerous advantages for separator applications. Unlike other single-loop logic solvers, the SLA can handle up to three safety loops simultaneously, making it more versatile and reducing the need for multiple logic solvers. With easy setup using PACTware software, easy-to-configure voting logic, and enhanced math capabilities, the SLA is an excellent solution for managing pressure protection in separators. By utilising these advanced features of the SLA, operators can increase operational efficiency, reduce risks, and maintain safety standards with ease and confidence, without requiring more complex and expensive equipment commonly associated with Safety PLCs.
Moore Industries
www.miinet.com
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