The design pressure of an NGL loading or unloading skid is derived from the vapor pressure of the most volatile blend it will handle, at the highest temperature the equipment can reach on site, plus the pressure contributions of pumps, compressors and thermal expansion. The safety equipment is then layered to cover that pressure envelope and the credible release scenarios. With LPG, the product definition is narrow enough to fix the design basis fairly easily. With NGL, the same skid may see batches with very different ethane content, so the design input is a composition range rather than a single product.
A skid gathers liquid and vapor lines, valves, safety devices, arm or hose connections and a local control panel into one pre-assembled package. Loading and unloading operations are covered on their own pages; this one deals with the package: its pressure basis, safety layers and plant interfaces.
Why the design pressure cannot come from a single product
Natural gas liquids are hydrocarbons that are gases at atmospheric pressure and remain liquid only under pressure. The more ethane in the mix, the higher the vapor pressure at a given temperature; the more butane and heavier material, the lower it is. If a skid may handle an ethane-lean batch one day and an ethane-rich batch the next, the pressure basis has to follow the most volatile blend. Contributions typically considered together are:
· Vapor pressure of the lightest blend: Calculated at the light end of the expected composition envelope and at the highest metal temperature on site, including solar gain.
· Pump shut-off head: The pressure a pump develops against a closed valve is added to its suction pressure.
· Compressor-driven unloading: Where vapor is compressed to push liquid out of the tanker, compressor discharge pressure and control limits are taken into account.
· Trapped liquid: Liquid blocked in between two closed valves pressurizes rapidly as it warms, so thermal relief is assessed for every isolatable section.
· Fast closure: Rapid ESD valve closure can create a pressure surge; closure time and sequence are chosen with this in mind.
The resulting design pressure sets flange ratings, valve body ratings, arm or hose working pressure, transmitter ranges and relief valve set points. The skid does not stand alone: together with the tanker and storage tank it forms one pressure system. Compatibility with the design pressure of every tanker and tank that will connect to it is therefore checked separately, and whether a vehicle built for LPG can carry an ethane-rich NGL batch must be confirmed from the tanker's own design data.
When pressure is released quickly, for example while depressurizing a line or when a relief valve lifts, flashing of light ends can chill the product and piping considerably. Because this effect can be stronger with ethane-rich blends, minimum design metal temperature is reviewed alongside the composition envelope.
Layers of protection
Safety on an NGL skid is not the job of a single device; it comes from passive and active layers that complement each other. Which functions are implemented as safety instrumented functions, and what safety integrity level (SIL) they need, is derived from a project-specific risk assessment rather than assumed.
Layer | Role on an NGL skid | Design focus |
Emergency shutdown (ESD) valves | Automatically isolate liquid and vapor lines at the skid inlet and outlet | Fail-safe position on loss of power, closure time, shutoff class |
Breakaway couplings | Separate the connection in a controlled way and close both sides if the tanker moves | Separate treatment of liquid and vapor lines, break load, maintenance access |
Pressure and thermal relief valves | Limit overpressure and expansion of trapped liquid | Discharge to a safe location or relief header, back pressure |
Excess flow and check valves | Limit reverse flow and uncontrolled release if a hose or line fails | Setting clear of normal flow, testability |
Interlocks | Prevent transfer until grounding is verified, the arm is off its rest and the vehicle is in position | Independence of interlock logic from the control system |
Gas and fire detection tie-in | Trips the ESD when a leak or fire is detected | Detector layout and interface with plant detection systems |
What stops, and in which order, on an ESD trip is defined in a cause-and-effect matrix: the pump or compressor stops, ESD valves close in a sequence that limits surge, the tanker side is isolated and the event is logged. Electrical equipment on the skid is selected with a protection method suited to the plant's hazardous area classification. Assembling individually conforming components does not remove the need to assess, at package level, any new ignition risks created by the combination.
What the package contains
Component group | Function | Standard / option |
Liquid line: strainer, isolation and control valves | Controlled transfer between tanker and plant | Standard |
Vapor line and isolation | Pressure balance between tanker and storage | Application-dependent; usually needed for closed transfer |
ESD, pressure relief and thermal relief valves | Protect the pressure envelope and isolation functions | Standard |
Arm or hose with breakaway couplings | Tanker connection | Per tanker connection type |
Local control panel and interlocks | Operator interface, permissives, alarms | Standard; plant control integration per project |
Meter and back-pressure control | Measurement of transferred quantity | Option; see NGL metering skids |
Sample connection | Pressurized sample for composition check | Option; may be required for custody transfer |
Pump or compressor | Drives the transfer | On-skid or separate, depending on layout |
Nitrogen purge connection | Hydrocarbon-freeing before maintenance | Option |
The skid-to-plant interface
A pre-assembled package performs well in the field only if its interfaces are defined early. On an NGL skid the relief header or flare connection deserves particular attention, since header back pressure affects relief valve type and setting. The technical specification also covers process connection ratings and materials, nitrogen and instrument air, power and grounding, signals exchanged with the plant ESD and gas detection systems, data for the control system, and when and where pressure and interlock function tests take place.
How TLY Enerji supports these projects
TLY Enerji provides engineering support for equipment selection and system design on NGL loading and unloading skid projects, starting from composition data and site conditions. Depending on scope, this can include supply of instruments, valves and pumps, configuration of PLC-based control and interlock logic, system integration, site installation, testing, commissioning, maintenance and technical support. Which parts of the package fall within TLY's scope of supply is agreed project by project through the technical specification.
NGL data needed to define the skid
· Expected composition envelope, especially maximum ethane and minimum heavy-end content
· Highest and lowest ambient temperatures on site and whether the skid is exposed to sun
· Design pressure and connection types of the tankers that will connect
· Design pressure of storage tanks and availability of a vapor line
· Loading only, unloading only or both; target transfer time and flow
· Availability and back pressure of a relief header or flare system
· Hazardous area classification and the plant's ESD and gas detection infrastructure
· Available utilities: nitrogen, instrument air, power
Related pages
· NGL truck loading systems: Where NGL loading departs from LPG loading in day-to-day operation.
· NGL truck unloading systems: Receiving NGL at the plant, pressure management and transfer method.
· LPG loading and unloading skids: Package scope for LPG, where the product definition is narrower.
· NGL metering skids: Choosing a meter and back-pressure control to add to the skid.
· ESD and safety interlocks for LPG terminals: General structure of safety functions in pressurized liquefied gas transfer.
Questions about NGL skids
Can a skid designed for LPG be used for NGL?
Only if the most volatile end of the NGL composition envelope stays within the skid's design pressure and material temperature limits. Because vapor pressure rises with ethane content, flange ratings, relief settings or hose pressure ratings chosen for LPG may fall short. Connected tankers and tanks need the same check, and the decision should rest on an assessment of the actual design data.
Why does ESD valve closure time matter?
A valve that closes too slowly lets more product escape during a leak. A valve that closes too quickly can generate a pressure surge in the liquid line and overload pipework, hoses or connections. Closure time and valve sequence are therefore set by looking at line length, flow rate and the strength of the connected equipment together.
If there is a breakaway coupling, are ESD valves still needed?
Yes. A breakaway coupling acts on mechanical separation, such as a tanker moving while still connected, and closes both sides to limit spillage. ESD valves isolate the lines on other triggers: gas detection, fire, operator action or a process upset. The two layers answer different scenarios and do not replace each other.
Can the SIL for the skid be fixed in advance?
Not as a general assumption. The safety integrity level is derived by risk assessment and depends on plant layout, inventory, operator presence and the other protection layers. The same skid may need different levels at two different plants. If the requirement is unclear early on, safety functions can be planned on an architecture that can be upgraded later.
What should be checked before switching to an ethane-rich batch?
First, whether the new blend's vapor pressure at the highest expected temperature stays within the design limits of the skid and connected equipment. Next, the back pressure and suction conditions needed to keep the meter and pump in liquid phase, relief valve settings and the chilling that depressurization could cause. Any change should also be reflected in operating procedures and control system settings.