Tier 3: Advanced control cages

In this latest instalment, we focus on our Tier 3 design, engineered for applications where precision control, noise attenuation and stability under demanding conditions are critical.

Building on the capabilities of Tier 2 ported cage designs, Tier 3 solutions represent a step up in control sophistication. They are specifically designed for severe service environments where standard trims can struggle to manage high pressure drops, noise and potential vibration. In this blog, we outline when it makes sense to move beyond ported designs. We will explain how drilled hole cages deliver enhanced flow characterisation, improved cavitation and noise control, and greater operational integrity. This ensures consistent, reliable performance across some of the most challenging process conditions.

The fundamental principle of a Tier 2 valve is simple: flow is regulated by progressively exposing a shaped opening as the plug strokes upwards. A Tier 3 valve, meanwhile, introduces a different architecture: the drilled hole cage, and a consequent step forward in both a more precise flow and in noise management.

Within the valve trim, the cylinder consists of a drilled hole cage containing many precisely positioned holes. As the plug rises through its stroke, multiple discrete holes are uncovered, and this incremental exposure provides greater flow control. Let’s consider a Tier 2 trim with four large ports, totalling 10 square inches of open area. Compare this to a Tier 3 cage with 1000 smaller holes, each measuring 0.01 in². Based on physical geometry, both trims have the same orifice flow. At 50% stroke, while both expose 5in2 of orifice, the hundred smaller openings of the drilled hole design engineer far greater control. That is not to say that a Tier 2 design cannot achieve a reasonable amount of range; it can. However, no Tier 2 trim can match the precision of the best Tier 3 design. When precision matters, a Tier 3 valve is the right choice.

The drilled hole cage offers a second, significant benefit in the form of improved noise management. While both Tier 2 and Tier 3 designs are single-stage, the sound power generated by each would be comparable. However, as the orifices in the drilled hole cage are smaller than those of a ported cage, the resultant peak frequency of the generated noise is higher.

Noise generated inside the valve must pass through the downstream pipe wall before it becomes a problem for plant operations. Pipe walls are highly effective at attenuating high-frequency sound. Therefore, the smaller the orifice, the higher the frequency of fluid passing through it, and the more the pipe can help reduce outside noise.

In practical terms, this means that as pressure changes, a drilled hole cage will produce a lower external noise level than a ported cage. When noise is a concern for the safety of your facility’s personnel, regulatory compliance, or site constraints, Tier 3 valves deliver performance that Tier 2 cannot match.

While the drilled hole cage is fundamentally a simple geometry - a cage is a cage - variations exist within that envelope, such as the following:

Characterised designs, where hole diameters increase progressively up the stroke and achieve lower noise at the lower stroke, high-pressure drop conditions;

Counterbored holes, which offer enhanced flow capacity without sacrificing control; and,

Greater flow characterisation that can be achieved by reducing the number of cage holes at the bottom and increasing them at the top.

Within our portfolio, the 840H and 860H series exemplifies the Tier 3 drilled hole cage design. Available in unbalanced plug configurations for body sizes between 1” and 3”, and balanced plug configurations from 1.5” to 24”, both offer rangeability of 30:1 to 50:1, with linear or equal percentage flow characteristics. Crucially, an infinite variety of hole patterns can be defined, enabling our engineers to create any customer flow profile your application requires.

At each IMI valve tier, your decision to move up the hierarchy should not be based on quality. Every IMI valve, from our Tier 1 butterfly to a Tier 6 additive-manufactured DRAG trim, is engineered to our exacting standards for precision and longevity. Instead, your decision depends on performance limits.

When the ceiling is reached with Tier 2, a Tier 3 valve can support:

Great flow control precision. When the requirement is for a tighter, more customisable flow profiling than a single-stage orifice can reliably deliver.

Noise control. Where external noise levels are a consideration or a constraint, the higher-frequency noise signature of a drilled hole cage, combined with pipe wall attenuation, delivers a lower outside noise level.

However, the drilled hole cage also has a boundary: pressure drop. As the pressure differential across a single-stage cage increases, two problems will occur.

First, noise. The velocity of the fluid passing through the orifice will reach sonic and then super-sonic, generating problematic noise levels. Second, loss of control. In liquid flow, the risk of cavitation increases. In gaseous flows, choked flow can develop. Both will cause limited flow control.

When pressure drops beyond what a single-stage design can handle, it becomes necessary to split the pressure drop into multiple steps. Determining how many steps are required will depend on varying factors, such as fluid vapour pressure, molecular weight and compressibility. That is when a Tier 4 valve becomes a consideration.

At IMI, our approach to Tier 3 – and the step up to Tier 4 – mirrors the one we apply across our entire portfolio. We will work with you to develop right-sized engineering that will meet your exact challenge. Our 840/860H series has been developed to achieve the required flow characteristic for the job, backed by decades of experience.

Our engineers work with you to determine whether the additional precision and noise control of a Tier 3 drilled hole cage will be the most applicable and cost-effective long-term option. We also take considerable care to ensure we can configure it to precisely meet your requirements.

IMI’s portfolio of control valve technologies.