Solvent Truck Unloading Systems

When volatile solvents are unloaded, cavitation and vapor are two sides of the same physical effect: once the pressure in the suction line approaches the solvent's vapor pressure, the liquid starts to vaporize inside the line. The result is cavitation at the pump inlet, false readings at the meter because of gas, and an uncontrolled vapor exchange between the truck and the receiving tank. The solution is pursued on three fronts: a suction line that leaves the pump an adequate pressure margin, a vapor balance line connecting the vapor spaces of the truck and the storage tank, and separating the gas that arrives at the end of unloading from the measurement.

The general arrangement of unloading pump, gas eliminator and quantity verification is covered on the truck unloading systems page. This page focuses on the decisions that change because a solvent is volatile, flammable and often purity-sensitive.

Why does solvent vaporize in the suction line?

For a pump to run without cavitation, the absolute pressure at its suction must stay a certain margin above the liquid's vapor pressure at that temperature; this margin is expressed as net positive suction head (NPSH). When pressure falls below vapor pressure, vapor bubbles form, collapse on the impeller, erode it and reduce discharge performance. With high-vapor-pressure solvents this margin is used up far more easily than with products such as water or diesel.

Typical factors that erode the suction margin when unloading solvents include:

·         Product temperature: Solvent in a truck standing in the sun warms up and its vapor pressure rises.

·         Long or narrow suction hose: Every meter of hose and every elbow adds friction loss and lowers pressure at the pump inlet.

·         Suction strainer: Pressure loss increases as it fouls, so differential pressure should be monitored.

·         Pump installed above the truck outlet: Static suction lift directly reduces the available margin.

·         Final stage of unloading: As the truck empties, the liquid level drops and air or vapor begins to enter the line.

For these reasons the pump at a solvent unloading point is placed as close to and as low as practical relative to the truck, the suction line is kept short and generously sized, and strainers are selected with the suction margin in mind. Reducing the flow rate toward the end of unloading is a simple way to limit vapor entering the line.

Vapor balancing: connecting two vapor spaces

As the truck empties, the space left in the compartment has to be filled with air or vapor, while at the same time the receiving storage tank fills and pushes its vapor out. In pump unloading, a vapor return line sends the vapor leaving the storage tank back to the truck. The truck is not exposed to vacuum, the tank is not pressurized, and solvent vapor is not released to atmosphere.

A blocked vapor return line can lead to overpressure in the storage tank and vacuum in the truck. Industry guidance therefore requires positive confirmation of flow in the vapor line during vapor-balanced unloading. Without vapor return, the truck must be vented from the top, and because a truck is not a process vessel, the risk of vacuum collapse has to be considered. In that case the solvent vapor leaving the storage tank can be routed to a scrubber or a vapor recovery unit, depending on the product.

If pressure unloading is used

Pressurizing the truck with gas to discharge it is used for some chemicals. Compressed air must not be used with a flammable solvent, since blowing air into a flammable atmosphere is not acceptable; an inert gas is needed instead. At the end of the discharge, gas passes through the lines to the receiving tank and can affect the meter, and the truck must be depressurized to atmospheric pressure before it leaves. With pump unloading, stopping the pump stops the flow immediately, which is a significant safety advantage in an emergency.

Static electricity applies to unloading too

Static risk is not limited to loading. During unloading, charge is generated in the pump and filter and carried into the receiving tank. The truck, hose and unloading skid are grounded before connection, and the pump does not start until the ground is verified. Electrical continuity of hoses is checked periodically. Terminating the tank inlet pipe below the liquid level prevents a low-conductivity solvent from free-falling and splashing inside the tank.

Product acceptance: before the wrong product reaches the tank

At a solvent facility, accepting the wrong product can spoil the purity of an entire tank. Acceptance control therefore starts before the unloading hose is connected:

·         Documents: The delivery note, certificate of analysis and, where required, the truck cleaning certificate are checked against the order and the destination tank.

·         Labeled and locked unloading points: Each inlet is labeled with the product name; no connection is made unless the driver holds a document showing the tank number.

·         Product-specific connections: Different coupling types or dedicated hoses for different solvents reduce the risk of mixing.

·         Sampling: If a sample is needed, it is taken by authorized site personnel with grounding in place.

Which quality parameters are checked on acceptance is set by the sales contract and the product specification. The site operating procedure should also state whether product is transferred to a quarantine tank or held in the truck while results are pending.

Design criteria

TLY Enerji's role

At solvent unloading points, TLY Enerji can support the selection and supply of pumps, meters, gas eliminators and valves based on process data, the integration of permissives such as grounding and vapor line flow confirmation into the control system, and installation, testing and commissioning. Whether a vapor recovery unit or storage tank equipment is included is defined as the scope boundary at the start of the project.

Data needed for your unloading point

·         Solvents to be unloaded, their vapor pressure and the highest expected product temperature

·         Truck type, number of compartments and position of the discharge connection

·         Target unloading time or flow rate

·         Intended pump location and elevation differences to the truck and storage tank

·         Venting, vapor return and nitrogen blanketing status of the storage tank

·         Quantity basis: meter, weighbridge or tank gauging

·         Acceptance procedure, sampling and quality control expectations

Related pages

·         Solvent vapor recovery and vapor return systems: How a vapor balance line differs from vapor treatment units.

·         Grounding and overfill prevention systems: Ground verification and the permissive chain, which apply to unloading as well.

·         Truck unloading systems: Unloading pump, gas eliminator and tank reconciliation.

·         Solvent metering skids: Meter selection for measuring the received quantity.

·         Chemical truck unloading systems: Unloading methods for different chemicals and interlocks against unloading into the wrong tank.

Questions about solvent unloading

What does noise and vibration from the pump indicate during solvent unloading?

Usually it means vapor is forming on the suction side, in other words cavitation. The solvent may have warmed up, the suction strainer may be fouled, or the truck may be nearly empty. The first things to check are suction pressure, strainer differential pressure and product temperature. If the problem keeps returning, the pump position and the diameter and length of the suction line should be reassessed.

Can solvent be unloaded without a vapor return line?

It can, but the truck compartment must then be vented from the top and the vapor leaving the storage tank has to be handled separately. If venting is inadequate, the truck can be damaged by vacuum. For volatile and odorous solvents, releasing vapor directly to atmosphere may also be unacceptable under environmental regulations, so vapor return and vapor treatment options are assessed together.

Will the meter read correctly if the truck is unloaded under nitrogen pressure?

For most of the discharge the meter sees liquid, but in the final stage nitrogen enters the line. Without a gas eliminator, or with one that is undersized, the meter may count that gas as well. For pressure unloading, a gas eliminator, end-of-discharge detection and stopping the meter when gas arrives therefore need to be part of the design.

Does a high-purity solvent need its own unloading line?

Where purity requirements are high, a line with a dedicated hose, pump and meter keeps the risk of mixing and cleaning to a minimum. If the line is shared, it should be defined in writing which products may use the same line, how the line is drained at changeover and how the first quantity received is checked.