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Direct answer
Choose a self-supporting open-top onion tank when the work requires direct access to the water surface—for example, visual checking, dipping, or positioning a pump intake from above. Choose a pillow tank when the water should remain inside an enclosed flexible body and filling or withdrawal can happen through specified fittings. Neither shape is automatically “better.” The right choice depends on how people and equipment interact with the water, the available footprint, capacity, fittings and site controls.
Choose an onion tank when direct surface access is part of the job
The defining feature of the ZolinFlex Onion Water Tank is its open top. Its self-supporting wall rises as the tank fills, leaving the water surface accessible from above.
That format is useful when the operation needs to see or reach the water directly. A team may need to monitor the level without a separate gauge, lower a temporary pump intake, fill from an overhead hose, or draw water with an open-access method. The tank can also act as a temporary receiving point before water moves to another part of the site.
Open access also changes the site-control question. If the project requires the stored water to be isolated from people, animals, wind-blown debris or rainfall, the buyer must define how that exposure will be managed. Do not assume an open-top tank includes a cover or that a cover suits every operation. State the requirement during specification.
Choose a pillow tank when the system should operate through fittings
A pillow tank is a closed-body flexible bladder that rests on a prepared surface and changes profile as it fills. Water enters and leaves through the selected inlet, outlet, drain or valve arrangement rather than through an open top.
This format suits projects where operators can connect the fill and withdrawal equipment to known fittings and do not need routine direct access to the water surface. ZolinWorks offers pillow water tanks across several public capacities, but a catalogue capacity alone does not complete the selection. The permitted footprint, filled height, liquid, handling method and connection layout still need project review.
The enclosed format should not be treated as proof of drinking-water suitability, pressure performance or compatibility with every liquid. Those are separate material and project questions. This comparison addresses the physical access method only.
Compare the formats with five site questions
| Site question | Open-top onion tank | Pillow tank |
|---|---|---|
| How will users reach the water? | Directly from above when the site procedure allows it. | Through specified fittings and connected equipment. |
| Must the water surface be enclosed? | No; the surface is open unless a separate approved control is specified. | The flexible body is enclosed; interfaces are through fittings. |
| How will the tank be filled and emptied? | Overhead/open filling or a specified connection; direct pump access may be possible. | Inlet, outlet, drain and valve positions should match the site system. |
| What space controls the choice? | Confirm the operating diameter, access around the rim and any exclusion area. | Confirm the full footprint, filled height and clearance around fittings. |
| What must appear in the quotation brief? | Capacity, intended open access, fill/withdrawal method, site controls, quantity and destination. | Capacity or permitted envelope, liquid, fittings, connection locations, quantity and destination. |
Answering those questions normally narrows the format before detailed material or commercial review begins.
Do not rely on the product name alone
Flexible-tank terminology is not perfectly consistent across suppliers. Onion tank, bladder tank and pillow tank may be used differently in catalogues. A name therefore is not enough for procurement.
Ask the supplier to confirm whether the proposed product is open at the top or fully enclosed, how it supports itself when filled, and where every required connection sits. A simple drawing or marked photograph is more useful than arguing over a label. If the project already has hoses, pumps or couplings, include their connection details and use the flexible water tank fittings guide to describe the interface clearly.
Prepare one format-selection brief for quotation
Before contacting the factory, collect the information that determines the access method and the configuration:
- intended application and liquid;
- target capacity, or the maximum available footprint and filled height;
- whether direct access to the water surface is required, permitted or prohibited;
- how water will arrive and leave—open fill, pump intake, gravity outlet or connected hose;
- inlet, outlet, drain, valve and connection standards already known;
- ground conditions and the clearance available around the operating tank;
- estimated quantity, destination market and required timing;
- any cover, barrier, labelling or handling requirement that needs project review.
Send the known details through the ZolinWorks project enquiry form. Unknowns can be listed as unknowns. The first goal is to confirm the correct open-top or enclosed format before material, fittings, final dimensions and commercial terms are fixed.